libxfs-5.15-sync_2021-09-17

xfs: sync libxfs with 5.15

Backport libxfs changes for 5.15.  The xfs_buf changes and the reworking
of the function predicates made things kind of messy, so I'm sending my
version of this to the list for evaluation so that Eric doesn't have to
stumble around wondering what I was smoking... ;)
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