linux: zfs: ctldir: set [amc]time to snapshot's creation property

If looking up a snapdir inode failed, hold pool config – hold the
snapshot – get its creation property – release it – release it,
then use that as the [amc]time in the allocated inode. If that
fails then fall back to current time. No performance impact since
this is only done when allocating a new snapdir inode.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #15110
Closes #15117