Fix ZFS on FreeBSD so that snapshots under .zfs/snapshot are NFS visible

[rmacklem]

Make the snapshots under .zfs/snapshot visible to NFS clients

Motivation and Context

Without this patch, newer versions of FreeBSD (post 13.2) will not allow NFS
access to snapshots under .zfs/snapshot

Description

Add an argument to mount_snapshot() which is the parent mount and
then have mount_snapshot() call vfs_exjail_clone() to fill in the mnt_exjail
field of the new mount created by mount_snapshot().

How Has This Been Tested?

A FreeBSD system without the change was NFS mounted by a client and
was not able to access a snapshot under .zfs/snapshot. Once the FreeBSD
system was patched, a client that NFS mounted the was able to
access the snapshot under .zfs/snapshot.

Types of changes

• [x ] Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.