net: rds: Fix possible NULL-pointer dereference

In rds_rdma_cm_event_handler_cmn() check, if conn pointer exists
before dereferencing it as rdma_set_service_type() argument

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: fd261ce6 ("rds: rdma: update rdma transport for tos")
Signed-off-by: Artem Chernyshev <artem.chernyshev@red-soft.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>