nfsd4: reject "negative" acl lengths

commit 64a817cfbded8674f345d1117b117f942a351a69 upstream.

Since we only enforce an upper bound, not a lower bound, a "negative"
length can get through here.

The symptom seen was a warning when we attempt to a kmalloc with an
excessive size.

Reported-by: Toralf Förster <toralf.foerster@gmx.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>