ocfs2: simplify ocfs2_invalidatepage() and ocfs2_releasepage()

Ocfs2 doesn't do data journalling.  Thus its ->invalidatepage and
->releasepage functions never get called on buffers that have journal
heads attached.  So just use standard variants of functions from
buffer.c.

Signed-off-by: Jan Kara <jack@suse.cz>
Cc: Joel Becker <jlbec@evilplan.org>
Cc: Mark Fasheh <mfasheh@suse.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>