What's the test coverage for this?  xfstest generic/192 tests that
atime is persisted over remounts, which we had a bug with when XFS
used to have a lazy atime implementation somewhat similar to the
proposal.

We should have something similar for c/mtime as well.  Also a test to
ensure timestamps are persisted afer a fsync, although right now I can't
imagine how to do that genericly as no other filesystem seems to have
an equivaent to XFS_IOC_GOINGDOWN.

It seems you also handle i_version updates lazily. although that's
not mentioned anywhere.  I actually have a clarification request out on
the IETF NFSv4 list about the persistance requirements for the change
counter but I've not seen an answer to it yet.