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: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Mingming Cao <cmm@us.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>