Rather than doing a terrible credential swapping hack, we just
check that the thing being mounted is a snapshot, and the mountpoint
is the zfsctl directory, then we allow it.

If the mount attempt is from inside a jail, on an unjailed dataset
(mounted from the host, not by the jail), the ability to mount the
snapshot is controlled by a new per-jail parameter: zfs.mount_snapshot

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Co-authored-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Allan Jude <allan@klarasystems.com>
Sponsored-by: Modirum MDPay
Sponsored-by: Klara Inc.
Closes openzfs#13758

When mounting the root filesystem, vfs_t->mnt_vnodecovered is null

This will cause zfsctl_is_node() to dereference a null pointer when
mounting, or updating the mount flags, on the root filesystem, both
of which happen during the boot process.

Reported-by: Martin Matuska <mm@FreeBSD.org>
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Signed-off-by: Allan Jude <allan@klarasystems.com>
Closes openzfs#14218