nfs4: fix channel attribute sanity-checks

The sanity checks here are incorrect; in the worst case they allow
values that crash the client.

They're also over-reliant on the preprocessor.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>