Fix the NFSv4 client hndling of a stale write verifier in the Commit operation.

When the NFSv4 client Commit operation encountered a stale write verifier,
it erroneously mapped that to EIO. This could have caused recently written
data to be lost when a server crashes/reboots between an UNSTABLE write
and the subsequent commit. This patch fixes this.
The bug was only for the NFSv4 client and did not affect NFSv3.

Tested by: cperciva
PR: 215887
MFC after: 2 weeks