xfs: refactor incore inode walking

This series prepares the incore inode walking code used by the
eofblocks/cowblocks scanner to handle deferred inode inactivation.
First we clean up the eofblocks/cowblocks incore inode walking code to
get rid of some of the warts left by reflink development.  Next, we
rip out the many trivial wrapper functions that don't add much value.
Finally, we refactor the various helpers and predicate functions to
reduce open-coded logic.