ocfs2: Wrap journal_access/journal_dirty for xattr buckets.

A common action is to call ocfs2_journal_access() and
ocfs2_journal_dirty() on the buffer heads of an xattr bucket.  Let's
create nice wrappers.

While we're there, let's drop the places that try to be smart by writing
only the first and last blocks of a bucket.  A bucket is contiguous, so
writing the whole thing is actually more efficient.

Signed-off-by: Joel Becker <joel.becker@oracle.com>
Signed-off-by: Mark Fasheh <mfasheh@suse.com>