xfs: hoist inode operations to libxfs

This series hoists inode creation, renaming, and deletion operations to
libxfs in anticipation of the metadata inode directory feature, which
maintains a directory tree of metadata inodes.  This will be necessary
for further enhancements to the realtime feature, subvolume support.

There aren't supposed to be any functional changes in this intense
refactoring -- we just split the functions into pieces that are generic
and pieces that are specific to libxfs clients.  As a bonus, we can
remove various open-coded pieces of mkfs.xfs and xfs_repair when this
series gets to xfsprogs.

If you're going to start using this mess, you probably ought to just
pull from my git trees.  The kernel patches[1] should apply against
4.20.  xfsprogs[2] and xfstests[3] can be found in their usual
places.  The git trees contain all four series' worth of changes.