RDMA/cma: Remove unneeded locking for req paths

The REQ flows are concerned that once the handler is called on the new
cm_id the ULP can choose to trigger a rdma_destroy_id() concurrently at
any time.

However, this is not true, while the ULP can call rdma_destroy_id(), it
immediately blocks on the handler_mutex which prevents anything harmful
from running concurrently.

Remove the confusing extra locking and refcounts and make the
handler_mutex protecting state during destroy more clear.

Link: https://lore.kernel.org/r/20200723070707.1771101-4-leon@kernel.org


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