NFSv4 referrals broken when not enabling junction support

[Salvatore Bonaccorso <carnil@debian.org>]

Hi Steve, hi linux-nfs people,

it got reported twice in Debian that  NFSv4 referrals are broken when
junction support is disabled. The two reports are at:

https://bugs.debian.org/1035908
https://bugs.debian.org/1083098

While arguably having junction support seems to be the preferred
option, the bug (or maybe unintended behaviour) arises when junction
support is not enabled (this for instance is the case in the Debian
stable/bookworm version, as we cannot simply do such changes in a
stable release; note later relases will have it enabled).

The "breakage" seems to be introduced with 15dc0bead10d ("exportd:
Moved cache upcalls routines  into libexport.a"), so
nfs-utils-2-5-3-rc6 as this will mask behind the #ifdef
HAVE_JUNCTION_SUPPORT's code which seems needed to support the refer=
in /etc/exports.

I had a quick conversation with Cuck offliste about this, and I can
hopefully state with his word, that yes, while nfsref is the direction
we want to go, we do not want to actually disable refer= in
/etc/exports.

Steve, what do you think? I'm not sure on the best patch for this,
maybe reverting the parts masking behind #ifdef HAVE_JUNCTION_SUPPORT
which are touched in 15dc0bead10d would be enough?

Thanks a lot already in advance!

Regards,
Salvatore