zfs: handle the `UF_NOUNLINK` chflags fflag

[ngie]

Summary

Prior to this change chflags uunlink <foo> would fail on FreeBSD with
EOPNOTSUPP since the necessary translation code had not been added for
UF_NOUNLINK. This change adds the missing glue for UF_NOUNLINK.

PR: 282981

Test Plan

Before

% chflags uunlink foo 
chflags: foo: Operation not supported
% chflags sunlink foo
chflags: foo: Operation not permitted
% sudo chflags sunlink foo
% uname -a
FreeBSD gilot.local 14.1-RELEASE-p4 FreeBSD 14.1-RELEASE-p4 GENERIC amd64

After

fbsd-current# mdconfig -a -t malloc -s 100m
md0
fbsd-current# mdconfig -a -t malloc -s 100m
md1
fbsd-current# zpool create howitzer md0 md1
fbsd-current# cd /howitzer
fbsd-current# : > foo
fbsd-current# chflags uunlink foo
fbsd-current# ls -l
total 1
-rw-r--r--  1 root wheel 0 Nov 29 10:44 foo
fbsd-current# rm foo
rm: foo: Operation not permitted
fbsd-current# chflags nouunlink foo
fbsd-current# rm foo
fbsd-current#

[emaste]

This should go to https://github.com/openzfs/zfs

[ngie]

As noted elsewhere, this unfortunately doesn't work for UID != 0: https://github.com/openzfs/zfs/pull/16820#issuecomment-2508540991 .

[ngie]

Will go through openzfs first.