RDMA/cma: Using the standard locking pattern when delivering the removal event

Whenever an event is delivered to the handler it should be done under the
handler_mutex and upon any non-zero return from the handler it should
trigger destruction of the cm_id.

cma_process_remove() skips some steps here, it is not necessarily wrong
since the state change should prevent any races, but it is confusing and
unnecessary.

Follow the standard pattern here, with the slight twist that the
transition to RDMA_CM_DEVICE_REMOVAL includes a cma_cancel_operation().

Link: https://lore.kernel.org/r/20200723070707.1771101-3-leon@kernel.org


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