ocfs2: Dirty the entire bucket in ocfs2_bucket_value_truncate()

ocfs2_bucket_value_truncate() currently takes the first bh of the
bucket, and magically plays around with the value bh - even though
the bucket structure in the calling function already has it.

In addition, future code wants to always dirty the entire bucket when it
is changed.  So let's pass the entire bucket into this function, skip
any block reads (we have them), and add the access/dirty logic.

ocfs2_xattr_update_value_size() is no longer necessary, as it only did
one thing other than journal access/dirty.

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