xfs: create libxfs helper to link a new inode into a directory

xfs: create libxfs helper to link a new inode into a directory

Create a new libxfs function to link a newly created 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>