xfs_io: add fiemap command to xfs_io

xfs_io: add fiemap command to xfs_io

Add a fiemap command that works almost exactly like bmap, but works on all
filesystem supporting the FIEMAP ioctl.  It is formatted similarly and
takes similar flags, the only thing thats different is obviously it doesn't
pit out AG info and it doesn't make finding prealloc space optional.

Signed-off-by: Josef Bacik <josef@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alex Elder <aelder@sgi.com>