ocfs2: Pull ocfs2_xattr_value_buf up into ocfs2_xattr_value_truncate().

Place an ocfs2_xattr_value_buf in ocfs2_xattr_value_truncate() and pass
it down to ocfs2_xattr_shrink_size().  We can also pass it into
ocfs2_xattr_extend_allocation(), replacing its ocfs2_xattr_value_buf.

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