We also try to make better use of the fs flags instead of
trying adapt the code according to the fs structures. In
the case of subsecond timestamps and birthtime we now
check that the feature is explicitly enabled: previously
we only checked that the reserved space was available and
silently wrote them.
This approach is much safer, especially if the filesystem
happens to use embedded inodes or support EAs.
Discussed with: Zheng Liu
MFC after: 5 days
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