xfs: create libxfs helper to link an existing inode into a directory

xfs: create libxfs helper to link an existing inode into a directory

Source kernel commit: c1f0bad4232fd309b2fe849153fcf473e775b1f7

Create a new libxfs function to link an existing inode into a directory.
The upcoming metadata directory feature will need this to create a
metadata directory tree.

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