RDMA/mlx5: Fix prefetch memory leak if get_prefetchable_mr fails

RDMA/mlx5: Fix prefetch memory leak if get_prefetchable_mr fails

destroy_prefetch_work() must always be called if the work is not going
to be queued. The num_sge also should have been set to i, not i-1
which avoids the condition where it shouldn't have been called in the
first place.

Cc: stable@vger.kernel.org
Fixes: 305d1d7ac95b ("RDMA/mlx5: Use SRCU properly in ODP prefetch")
Link: https://lore.kernel.org/r/20200727095712.495652-1-leon@kernel.org
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>