IB/srp: Fix srp_create_target() error handling

Avoid that the following kernel oops occurs if memory pool
allocation fails:

BUG: unable to handle kernel NULL pointer dereference at (null)
IP: [<ffffffffa048d0a0>] ib_drain_rq+0x0/0x20 [ib_core]
Call Trace:
 [<ffffffffa04af386>] srp_create_target+0xca6/0x13a9 [ib_srp]
 [<ffffffff813cc863>] dev_attr_store+0x13/0x20
 [<ffffffff81214b50>] sysfs_kf_write+0x40/0x50
 [<ffffffff81213f1c>] kernfs_fop_write+0x13c/0x180
 [<ffffffff81197683>] __vfs_write+0x23/0xf0
 [<ffffffff81198744>] vfs_write+0xa4/0x1a0
 [<ffffffff81199a44>] SyS_write+0x44/0xa0
 [<ffffffff8159e3e9>] entry_SYSCALL_64_fastpath+0x1c/0xac

Fixes: 1dc7b1f1 ("IB/srp: use the new CQ API")
Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Tested-by: Laurence Oberman <loberman@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Sagi Grimberg <sagi@grimberg.me>
Cc: <stable@vger.kernel.org> # v4.5+
Signed-off-by: Doug Ledford <dledford@redhat.com>