This is much simpler than for ffs since there are many fewer places
where we need to choose between a delayed write and a sync write --
just 5 in msdosfs and more than 30 in ffs.
This is more complete and correct than in ffs. Several places in ffs
are are still missing the choice. ffs_update() has a layering violation
that breaks callers which want to force a sync update (mainly fsync(2)
and O_SYNC write(2)).
However, fsync(2) and O_SYNC write(2) are still more broken than in
ffs, since they are broken for default (non-sync non-async) mounts
too. Both fail to sync the FAT in all cases, and both fail to sync
the directory entry in some cases after losing a race. Async everything
is probably safer than the half-baked sync of metadata given by default
mounts.
c590272b42