RDMA/mlx5: Fix locking in MR cache work queue

RDMA/mlx5: Fix locking in MR cache work queue

All of the members of mlx5_cache_ent must be accessed while holding the
spinlock, add the missing spinlock in the __cache_work_func().

Using cache->stopped and flush_workqueue() is an inherently racy way to
shutdown self-scheduling work on a queue. Replace it with ent->disabled
under lock, and always check disabled before queuing any new work. Use
cancel_work_sync() to shutdown the queue.

Use READ_ONCE/WRITE_ONCE for dev->last_add to manage concurrency as
coherency is less important here.

Split fill_delay from the bitfield. C bitfield updates are not atomic and
this is just a mess. Use READ_ONCE/WRITE_ONCE, but this could also use
test_bit()/set_bit().

Link: https://lore.kernel.org/r/20200310082238.239865-11-leon@kernel.org
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>