nfscl: Fix setting of mtime for the NFSv4.n client

It was reported on freebsd-fs@ that unrolling a tarball
failed to set the correct modify time if delegations
were being issued.

This patch fixes the problem.

This bug only affects NFSv4 mounts where delegations
are being issued. Not running the nfscbd or disabling
delegations on the NFSv4 server avoids the problem.

Reported by: Peter Much <pmc@citylink.dinoex.sub.org>
Tested by: Peter Much <pmc@citylink.dinoex.sub.org>
MFC after: 3 days