RDMA/mlx5: Fix potential race between destroy and CQE poll

The SRQ can be destroyed right before mlx5_cmd_get_srq is called.
In such case the latter will return NULL instead of expected SRQ.

Fixes: e126ba97 ("mlx5: Add driver for Mellanox Connect-IB adapters")
Link: https://lore.kernel.org/r/20200830084010.102381-5-leon@kernel.org


Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>