xfs_spaceman: Free space mapping command

xfs_spaceman: Free space mapping command

Add freespace mapping tool modelled on the xfs_db freesp command.
The advantage of this command over xfs_db is that it can be done
online and is coherent with concurrent modifications to the
filesystem.

This requires the kernel to support the XFS_IOC_GETFSMAP ioctl to map
free space indexes.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
[darrick: port from FIEMAPFS to GETFSMAP]
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>