jffs2: use mutex_is_locked() in __jffs2_flush_wbuf()

Use a helper to test if a mutex is held instead of a hack with
mutex_trylock().
Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Artem Bityutskiy <dedekind1@gmail.com>

fs/jffs2/wbuf.c

@@ -578,8 +578,7 @@ static int __jffs2_flush_wbuf(struct jffs2_sb_info *c, int pad)
 	if (!jffs2_is_writebuffered(c))
 		return 0;
 
-	if (mutex_trylock(&c->alloc_sem)) {
-		mutex_unlock(&c->alloc_sem);
+	if (!mutex_is_locked(&c->alloc_sem)) {
 		printk(KERN_CRIT "jffs2_flush_wbuf() called with alloc_sem not locked!\n");
 		BUG();
 	}