zfs: fix mounting snapshots in .zfs/snapshot as a regular user

This is a cherry-pick from vendor/openzfs/master resulting
in a direct commit to stable/13

The cherry-pick to branch zfs-2.1-release has been proposed upstream.

Vendor pull requeset merges:

#13758 Allow mounting snapshots in .zfs/snapshot as a regular user
#14218 Bugfix: Avoid a null pointer dereference in zfs_mount() on FreeBSD

PR: 260160

(cherry picked from commit 595d3ac2ed61331124feda2cf5787c3dd4c7ae09)
(cherry picked from commit d27a00283faf4ec2b997ff2376dee4a080e1ca7b)