staging: lustre: selftest: fix _recycle_buffer context imbalance

Fix the sparse warning about a context imbalance in the
srpc_service_recycle_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:725:20: warning: context imbalance in 'srpc_service_recycle_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

@@ -698,6 +698,7 @@ srpc_finish_service(struct srpc_service *sv)
 /* called with sv->sv_lock held */
 static void
 srpc_service_recycle_buffer(struct srpc_service_cd *scd, srpc_buffer_t *buf)
+	__must_hold(&scd->scd_lock)
 {
 	if (!scd->scd_svc->sv_shuttingdown && scd->scd_buf_adjust >= 0) {
 		if (srpc_service_post_buffer(scd, buf) != 0) {