dlm: remove redundant variable assignments

Once the tcp_create_listen_sock() is returned successfully, we
will invoke add_sock() immediately. In add_sock(), the 'con'
variable is assigned to 'sk_user_data', meanwhile, the 'sock' is
also set to 'con->sock'. So it's unnecessary to do the same thing
in tcp_create_listen_sock().

Signed-off-by: Xue Ying <ying.xue@windriver.com>
Signed-off-by: David Teigland <teigland@redhat.com>