xfs: switch imeta namespace operations to be relative to the parent

xfs: switch imeta namespace operations to be relative to the parent

Source kernel commit: a8ba89332674e0aa549507457b412927fb0fdd93

Switch the imeta lookup / create / link APIs to work on [parent inode,
pathname component] pairs instead of the multi-level xfs_imeta_path
structure.  This simplifies the code a lot, and makes the dependencies
on the parent directory (there is only one right one) more clear.

Note that the metapath code looks a bit off with this now, we might want
to try the same unlink/relink trick as we do for the leaf files there.

Signed-off-by: Christoph Hellwig <hch@lst.de>