jbd2: fix build when CONFIG_BUG is not enabled

Fix build error when CONFIG_BUG is not enabled:

fs/jbd2/transaction.c:1175:3: error: implicit declaration of function '__WARN'

by changing __WARN() to WARN_ON(), as suggested by
Arnaud Lacombe <lacombar@gmail.com>.
Signed-off-by: NRandy Dunlap <rdunlap@xenotime.net>
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Arnaud Lacombe <lacombar@gmail.com>

fs/jbd2/transaction.c

@@ -27,6 +27,7 @@
 #include <linux/highmem.h>
 #include <linux/hrtimer.h>
 #include <linux/backing-dev.h>
+#include <linux/bug.h>
 #include <linux/module.h>
 
 static void __jbd2_journal_temp_unlink_buffer(struct journal_head *jh);
@@ -1171,8 +1172,7 @@ int jbd2_journal_dirty_metadata(handle_t *handle, struct buffer_head *bh)
 	jbd_unlock_bh_state(bh);
 out:
 	JBUFFER_TRACE(jh, "exit");
-	if (ret)
-		__WARN();	/* All errors are bugs, so dump the stack */
+	WARN_ON(ret);	/* All errors are bugs, so dump the stack */
 	return ret;
 }