nfsd: Update the boot verifier on stable writes too.

We don't know if the error returned by the fsync() call is
exclusive to the data written by the stable write, so play it
safe.

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