nvmet-rdma: add unlikely check at nvmet_rdma_map_sgl_keyed

The calls to nvmet_req_alloc_sgl and rdma_rw_ctx_init should usually
succeed, so add this simple optimization to the fast path.

Signed-off-by: Israel Rukshin <israelr@mellanox.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Max Gurtovoy <maxg@mellanox.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>