RDMA/rtrs: server: main functionality

This is main functionality of rtrs-server module, which accepts set of
RDMA connections (so called rtrs session), creates/destroys sysfs entries
associated with rtrs session and notifies upper layer
(user of RTRS API) about RDMA requests or link events.

Link: https://lore.kernel.org/r/20200511135131.27580-11-danil.kipnis@cloud.ionos.com
Signed-off-by: Danil Kipnis <danil.kipnis@cloud.ionos.com>
Signed-off-by: Jack Wang <jinpu.wang@cloud.ionos.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>