OpenZFS: five filesystem tests fail under conf_zfs=openzfs (known gaps from #1423)

[gburd]

While finalizing #1423 (selectable in-kernel ZFS via upstream OpenZFS), five
existing OSv filesystem tests are known to fail when run against an OpenZFS root
under conf_zfs=openzfs. They are documented as known gaps in
modules/open_zfs/FINDINGS-osv-openzfs.md and tracked here for follow-up. The
ZFS feature itself (boot, pool import, root mount, read/write, and the Tier 0-3
feature matrices) is unaffected.

Test	Why it fails under OpenZFS
tst-fallocate	posix_fallocate/preallocation not implemented on the OpenZFS vnops path.
tst-fs-link	hard-link paths not fully wired through the OpenZFS zfs_link vnop on OSv.
tst-utimensat	utimensat timestamp update not reflected through the OpenZFS SA path.
tst-utimes	utimes/utimensat timestamp update, same root cause as tst-utimensat.
tst-zfs-mount	the in-guest mount/remount sequence the test drives is not fully supported by the OSv OpenZFS mount shim.

These are not regressions from #1423; they are unimplemented corners of the OSv
OpenZFS platform layer, left for follow-up.