xfs: create libxfs helper to remove an existing inode/name from a directory

xfs: create libxfs helper to remove an existing inode/name from a directory

Create a new libxfs function to remove a (name, inode) entry from 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>