RDMA/mlx5: Lock access to ent->available_mrs/limit when doing queue_work

Accesses to these members needs to be locked. There is no reason not to
hold a spinlock while calling queue_work(), so move the tests into a
helper and always call it under lock.

The helper should be called when available_mrs is adjusted.

Link: https://lore.kernel.org/r/20200310082238.239865-10-leon@kernel.org
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>