ocfs2: Provide ocfs2_xa_fill_value_buf() for external value processing

We use the ocfs2_xattr_value_buf structure to manage external values.
It lets the value tree code do its work regardless of the containing
storage.  ocfs2_xa_fill_value_buf() initializes a value buf from an
ocfs2_xa_loc entry.

Signed-off-by: Joel Becker <joel.becker@oracle.com>