fdescfs: improve linrdlnk mount option

Instead of using VV_READLINK vnode flag and checking it in one place,
just assign VLNK type to the Fdesc vnodes for linrdlnk mounts. Then all
places where symlinks needs to be followed, e.g. lookup(), are handled.

PR: 272127
Reported by: Peter Eriksson <pen@lysator.liu.se>
Reviewed by: markj
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D40700