jbd2: Fix reference counting on the journal commit block's buffer head

With journal checksum patch we added asynchronous commits of journal
commit headers, and accidentally dropped taking a reference on the
buffer head.

(Before the change, sync_dirty_buffer did the get_bh(). The associative
put_bh is done by journal_wait_on_commit_record().)
Signed-off-by: NAneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: NMingming Cao <cmm@us.ibm.com>
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>

fs/jbd2/commit.c

@@ -136,7 +136,7 @@ static int journal_submit_commit_record(journal_t *journal,
 
 	JBUFFER_TRACE(descriptor, "submit commit block");
 	lock_buffer(bh);
-
+	get_bh(bh);
 	set_buffer_dirty(bh);
 	set_buffer_uptodate(bh);
 	bh->b_end_io = journal_end_buffer_io_sync;