nfsd4: fix setclientid_confirm same_cred check

nfsd4: fix setclientid_confirm same_cred check

New clients are created only by nfsd4_setclientid(), which always gives
any new client a unique clientid.  The only exception is in the
"callback update" case, in which case it may create an unconfirmed
client with the same clientid as a confirmed client.  In that case it
also checks that the confirmed client has the same credential.

Therefore, it is pointless for setclientid_confirm to check whether a
confirmed and unconfirmed client with the same clientid have matching
credentials--they're guaranteed to.

Instead, it should be checking whether the credential on the
setclientid_confirm matches either of those.  Otherwise, it could be
anyone sending the setclientid_confirm.  Granted, I can't see why anyone
would, but still it's probalby safer to check.

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