RDMA/devices: Remove the lock around remove_client_context

Due to the complexity of client->remove() callbacks it is desirable to not
hold any locks while calling them. Remove the last one by tracking only
the highest client ID and running backwards from there over the xarray.

Since the only purpose of that lock was to protect the linked list, we can
drop the lock.

Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Link: https://lore.kernel.org/r/20190731081841.32345-3-leon@kernel.org
Signed-off-by: Doug Ledford <dledford@redhat.com>