knfsd: nfsd: use ip-address-based domain in secinfo case

With this patch, we fall back on using the gss/pseudoflavor only if we fail to
find a matching auth_unix export that has a secinfo list.

As long as sec= options aren't used, there's still no change in behavior here
(except possibly for some additional auth_unix cache lookups, whose results
will be ignored).

The sec= option, however, is not actually enforced yet; later patches will add
the necessary checks.

Signed-off-by: "J. Bruce Fields" <bfields@citi.umich.edu>
Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>