staging: lustre: selftest: fix sfw_deactivate_session context imbalance

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

  drivers/staging/lustre/lnet/selftest/framework.c:210:9: warning: context imbalance in 'sfw_deactivate_session' - unexpected unlock
Signed-off-by: NJeremiah Mahler <jmmahler@gmail.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

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

@@ -194,9 +194,9 @@ sfw_del_session_timer (void)
 	return EBUSY; /* racing with sfw_session_expired() */
 }
 
-/* called with sfw_data.fw_lock held */
 static void
 sfw_deactivate_session (void)
+	__must_hold(&sfw_data.fw_lock)
 {
 	sfw_session_t *sn = sfw_data.fw_session;
 	int	    nactive = 0;