mount(8): Avoid truncation when fstab-formatting unionfs mount info

When displaying unionfs mounts in fstab format (mount -p), mount(8)
currently uses strlcpy to remove the disposition prefix from the mount
name returned by getmntinfo(3). But strlcpy, like strcpy before it,
does not guarantee correct behavior if the source and destination
buffers overlap.

Just offset the buffer and avoid the destructive copy in the first
place.

PR: 283420
Reviewed by: imp (previous version), olce
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D48177