RDMA/ucma: Support write an event into a CM

RDMA/ucma: Support write an event into a CM

Enable user-space to inject an event into a CM through it's event
channel. Two new events are added and supported: RDMA_CM_EVENT_USER and
RDMA_CM_EVENT_INTERNAL. With these 2 events a new event parameter "arg"
is supported, which is passed from sender to receiver transparently.

With this feature an application is able to write an event into a CM
channel with a new user-space rdmacm API. For example thread T1 could
write an event with the API:
    rdma_write_cm_event(cm_id, RDMA_CM_EVENT_USER, status, arg);
and thread T2 could receive the event with rdma_get_cm_event().

Signed-off-by: Mark Zhang <markzhang@nvidia.com>
Reviewed-by: Vlad Dumitrescu <vdumitrescu@nvidia.com>
Link: https://patch.msgid.link/fdf49d0b17a45933c5d8c1d90605c9447d9a3c73.1751279794.git.leonro@nvidia.com
Signed-off-by: Leon Romanovsky <leon@kernel.org>