net: qmi_wwan: should hold RTNL while changing netdev type

net: qmi_wwan: should hold RTNL while changing netdev type

The notifier calls were thrown in as a last-minute fix for an
imagined "this device could be part of a bridge" problem. That
revealed a certain lack of locking.  Not to mention testing...

Avoid this splat:

RTNL: assertion failed at net/core/dev.c (1639)
CPU: 0 PID: 4293 Comm: bash Not tainted 4.4.0-rc3+ #358
Hardware name: LENOVO 2776LEG/2776LEG, BIOS 6EET55WW (3.15 ) 12/19/2011
 0000000000000000 ffff8800ad253d60 ffffffff8122f7cf ffff8800ad253d98
 ffff8800ad253d88 ffffffff813833ab 0000000000000002 ffff880230f48560
 ffff880230a12900 ffff8800ad253da0 ffffffff813833da 0000000000000002
Call Trace:
 [<ffffffff8122f7cf>] dump_stack+0x4b/0x63
 [<ffffffff813833ab>] call_netdevice_notifiers_info+0x3d/0x59
 [<ffffffff813833da>] call_netdevice_notifiers+0x13/0x15
 [<ffffffffa09be227>] raw_ip_store+0x81/0x193 [qmi_wwan]
 [<ffffffff8131e149>] dev_attr_store+0x20/0x22
 [<ffffffff811d858b>] sysfs_kf_write+0x49/0x50
 [<ffffffff811d8027>] kernfs_fop_write+0x10a/0x151
 [<ffffffff8117249a>] __vfs_write+0x26/0xa5
 [<ffffffff81085ed4>] ? percpu_down_read+0x53/0x7f
 [<ffffffff81174c9e>] ? __sb_start_write+0x5f/0xb0
 [<ffffffff81174c9e>] ? __sb_start_write+0x5f/0xb0
 [<ffffffff81172c37>] vfs_write+0xa3/0xe7
 [<ffffffff811734ad>] SyS_write+0x50/0x7e
 [<ffffffff8145c517>] entry_SYSCALL_64_fastpath+0x12/0x6f

Fixes: 32f7adf633b9 ("net: qmi_wwan: support "raw IP" mode")
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>