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

Call vfs_exjail_clone() for mounts created under .zfs/snapshot
to fill in the mnt_exjail field for the mount. If this is not
done, the snapshots under .zfs/snapshot with not be accessible
over NFS.

This version has the argument name in vfs.h fixed to match that
of the name in spl_vfs.c, although it really does not matter.

External-issue: https://reviews.freebsd.org/D42672
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Rick Macklem <rmacklem@uoguelph.ca>
Closes #15563