nfsd4: reject "negative" acl lengths

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>
Cc: stable@kernel.org
Signed-off-by: J. Bruce Fields <bfields@redhat.com>