xfs: return inode fork offset in bulkstat for fsr

So that fsr can attempt to get the fork offset of the temporary
inode it uses the same as the inode it is defragmenting, pass the
fork offset out in the bulkstat information.

The bulkstat structure has padding that has always been zeroed, so
userspace can tell if this field is set or not by use of the xattr
present flag and a non-zero value for the fork offset.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alex Elder <aelder@sgi.com>