[XFS] Export via a function xfs_buftarg_list for use by kdb/xfsidbg.

SGI-PV: 963465
SGI-Modid: xfs-linux-melb:xfs-kern:28414a

Signed-off-by: Tim Shimmin <tes@sgi.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>