ZFS: Allow setting file creation time

[asomers]

Summary

Currently creation time (birth time) on ZFS is read-only which is
contrary to the manual page of futimes.

The proposed change adopts similar semantics as UFS2, where VFS is responsible
for doing sanity checks (permissions, birth time can not go forward, etc.) and
use that information to update SA_ZPL_CRTIME (=di_birthtime for UFS).

Test Plan

This code have been tested on -CURRENT but I would like to see comments,
especially if this is actually a desirable behavior and/or if it's Okay
to use the unused bit on Illumos for AT_CRTIME.

[jilles]

The concept of allowing to set creation time looks good to me.

Adding AT_CRTIME may be wrong. Further down in sys/cddl/contrib/opensolaris/uts/common/sys/vnode.h, there is XAT_CREATETIME which should perhaps be used/extended instead.

[mckusick]

I think that allowing the setting of creation time for ZFS is a sensible addition. Files coming off of archive media have good reason to set it.

[asomers]

I independently made the same change in D9353

[asomers]

Fixed in r313483