IB/srpt: Fix the RDMA completion handlers

IB/srpt: Fix the RDMA completion handlers

Avoid that the following kernel crash is triggered when processing
an RDMA completion:

BUG: unable to handle kernel paging request at 0000000100000198
IP: [<ffffffff810a4ea2>] __lock_acquire+0xa2/0x560
Call Trace:
 [<ffffffff810a53c2>] lock_acquire+0x62/0x80
 [<ffffffff8151bd33>] _raw_spin_lock_irqsave+0x43/0x60
 [<ffffffffa04fd437>] srpt_rdma_read_done+0x57/0x120 [ib_srpt]
 [<ffffffffa0144dd3>] __ib_process_cq+0x43/0xc0 [ib_core]
 [<ffffffffa0145115>] ib_cq_poll_work+0x25/0x70 [ib_core]
 [<ffffffff8107184d>] process_one_work+0x1bd/0x460
 [<ffffffff81073148>] worker_thread+0x118/0x420
 [<ffffffff81078454>] kthread+0xe4/0x100
 [<ffffffff8151cbbf>] ret_from_fork+0x3f/0x70

Fixes: commit 59fae4deaad3 ("IB/srpt: chain RDMA READ/WRITE requests").
Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>