IB/srp: Remove unnecessary unlikely()

WARN_ON() already contains an unlikely(), so it's not necessary to wrap it
into another.

Signed-off-by: Igor Stoppa <igor.stoppa@huawei.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>