ocfs2: Pass value buf to ocfs2_xattr_update_entry().

ocfs2_xattr_update_entry() updates the entry portion of an xattr buffer.
This can be part of multiple metadata block types, so pass the buffer in
via an ocfs2_xattr_value_buf.

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