Honour mpsafe_vfs=0 in softdep_flush. This is basically a MFC of revision

1.206, except that Giant remains unconditionally acquired in the #ifdef
QUOTA case here (as QUOTA-enabled UFS on RELENG_6 is not MPSAFE).

Reviewed by:	kib

sys/ufs/ffs/ffs_softdep.c

@@ -728,6 +728,8 @@ softdep_flush(void)
 		kthread_suspend_check(softdepproc);
 #ifdef QUOTA
 		mtx_lock(&Giant);
+#else
+		vfslocked = VFS_LOCK_GIANT((struct mount *)NULL);
 #endif
 		ACQUIRE_LOCK(&lk);
 		/*
@@ -746,6 +748,8 @@ softdep_flush(void)
 		FREE_LOCK(&lk);
 #ifdef QUOTA
 		mtx_unlock(&Giant);
+#else
+		VFS_UNLOCK_GIANT(vfslocked);
 #endif
 		remaining = 0;
 		mtx_lock(&mountlist_mtx);