xfs: add parent pointer ioctls

xfs: add parent pointer ioctls

Source kernel commit: 233f4e12bbb2c5fb1588b857336a26e8bb6942af

This patch adds a pair of new file ioctls to retrieve the parent pointer
of a given inode.  They both return the same results, but one operates
on the file descriptor passed to ioctl() whereas the other allows the
caller to specify a file handle for which the caller wants results.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>