staging: lustre: selftest: fix _post_buffer context imbalance

Fix the sparse warning about a context imbalance in the
srpc_service_post_buffer function by telling sparse that it
should expect the lock to be held on entry and held on exit.

  drivers/staging/lustre/lnet/selftest/rpc.c:508:17: warning: context imbalance in 'srpc_service_post_buffer' - unexpected unlock
Signed-off-by: Jeremiah Mahler <jmmahler@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/lustre/lnet/selftest/rpc.c

@@ -468,6 +468,7 @@ srpc_post_passive_rqtbuf(int service, int local, void *buf, int len,
 
 static int
 srpc_service_post_buffer(struct srpc_service_cd *scd, struct srpc_buffer *buf)
+	__must_hold(&scd->scd_lock)
 {
 	struct srpc_service	*sv = scd->scd_svc;
 	struct srpc_msg		*msg = &buf->buf_msg;