[XFS] Hook up the fiemap ioctl.

This adds the fiemap inode_operation, which for us converts the
fiemap values & flags into a getbmapx structure which can be sent
to xfs_getbmap.  The formatter then copies the bmv array back into
the user's fiemap buffer via the fiemap helpers.

If we wanted to be more clever, we could also return mapping data
for in-inode attributes, but I'm not terribly motivated to do that
just yet.

Signed-off-by: Eric Sandeen <sandeen@sandeen.net>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Niv Sardi <xaiki@sgi.com>