vrf: vrf_master_ifindex_rcu is not always called with rcu read lock

vrf: vrf_master_ifindex_rcu is not always called with rcu read lock

While running net-next I hit this:
[  634.073119] ===============================
[  634.073150] [ INFO: suspicious RCU usage. ]
[  634.073182] 4.2.0-rc6+ #45 Not tainted
[  634.073213] -------------------------------
[  634.073244] include/net/vrf.h:38 suspicious rcu_dereference_check()
usage!
[  634.073274]
               other info that might help us debug this:

[  634.073307]
               rcu_scheduler_active = 1, debug_locks = 1
[  634.073338] 2 locks held by swapper/0/0:
[  634.073369]  #0:  (((&n->timer))){+.-...}, at: [<ffffffff8112bc35>]
call_timer_fn+0x5/0x480
[  634.073412]  #1:  (slock-AF_INET){+.-...}, at: [<ffffffff8174f0f5>]
icmp_send+0x155/0x5f0
[  634.073450]
               stack backtrace:
[  634.073483] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 4.2.0-rc6+ #45
[  634.073514] Hardware name: innotek GmbH VirtualBox/VirtualBox, BIOS
VirtualBox 12/01/2006
[  634.073545]  0000000000000000 0593ba8242d9ace4 ffff88002fc03b48
ffffffff81803f1b
[  634.073612]  0000000000000000 ffffffff81e12500 ffff88002fc03b78
ffffffff811003c5
[  634.073642]  0000000000000000 ffff88002ec4e600 ffffffff81f00f80
ffff88002fc03cf0
[  634.073669] Call Trace:
[  634.073694]  <IRQ>  [<ffffffff81803f1b>] dump_stack+0x4c/0x65
[  634.073728]  [<ffffffff811003c5>] lockdep_rcu_suspicious+0xc5/0x100
[  634.073763]  [<ffffffff8174eb56>] icmp_route_lookup+0x176/0x5c0
[  634.073793]  [<ffffffff8174f2fb>] ? icmp_send+0x35b/0x5f0
[  634.073818]  [<ffffffff8174f274>] ? icmp_send+0x2d4/0x5f0
[  634.073844]  [<ffffffff8174f3ce>] icmp_send+0x42e/0x5f0
[  634.073873]  [<ffffffff8170b662>] ipv4_link_failure+0x22/0xa0
[  634.073899]  [<ffffffff8174bdda>] arp_error_report+0x3a/0x80
[  634.073926]  [<ffffffff816d6100>] ? neigh_lookup+0x2c0/0x2c0
[  634.073952]  [<ffffffff816d396e>] neigh_invalidate+0x8e/0x110
[  634.073984]  [<ffffffff816d62ae>] neigh_timer_handler+0x1ae/0x290
[  634.074013]  [<ffffffff816d6100>] ? neigh_lookup+0x2c0/0x2c0
[  634.074013]  [<ffffffff8112bce3>] call_timer_fn+0xb3/0x480
[  634.074013]  [<ffffffff8112bc35>] ? call_timer_fn+0x5/0x480
[  634.074013]  [<ffffffff816d6100>] ? neigh_lookup+0x2c0/0x2c0
[  634.074013]  [<ffffffff8112c2bc>] run_timer_softirq+0x20c/0x430
[  634.074013]  [<ffffffff810af50e>] __do_softirq+0xde/0x630
[  634.074013]  [<ffffffff810afc97>] irq_exit+0x117/0x120
[  634.074013]  [<ffffffff81810976>] smp_apic_timer_interrupt+0x46/0x60
[  634.074013]  [<ffffffff8180e950>] apic_timer_interrupt+0x70/0x80
[  634.074013]  <EOI>  [<ffffffff8106b9d6>] ? native_safe_halt+0x6/0x10
[  634.074013]  [<ffffffff81101d8d>] ? trace_hardirqs_on+0xd/0x10
[  634.074013]  [<ffffffff81027d43>] default_idle+0x23/0x200
[  634.074013]  [<ffffffff8102852f>] arch_cpu_idle+0xf/0x20
[  634.074013]  [<ffffffff810f89ba>] default_idle_call+0x2a/0x40
[  634.074013]  [<ffffffff810f8dcc>] cpu_startup_entry+0x39c/0x4c0
[  634.074013]  [<ffffffff817f9cad>] rest_init+0x13d/0x150
[  634.074013]  [<ffffffff81f69038>] start_kernel+0x4a8/0x4c9
[  634.074013]  [<ffffffff81f68120>] ?
early_idt_handler_array+0x120/0x120
[  634.074013]  [<ffffffff81f68339>] x86_64_start_reservations+0x2a/0x2c
[  634.074013]  [<ffffffff81f68485>] x86_64_start_kernel+0x14a/0x16d

It would seem vrf_master_ifindex_rcu() can be called without RCU held in
other contexts as well so introduce a new helper which acquires rcu and
returns the ifindex.
Also add curly braces around both the "if" and "else" parts as per the
style guide.

Signed-off-by: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>