fix against credits leak in journal_release_buffer()

The idea is to charge a buffer in journal_dirty_metadata(), not in
journal_get_*_access()).  Each buffer has flag call
journal_dirty_metadata() sets on the buffer.
Signed-off-by: Alex Tomas <alex@clusterfs.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>