allocate and use a single verifier instance instead of creating a new on...

...e for every write/commit

nfs3 rfc says the verifier is fixed for the duration of a server's lifetime and should change on server restarts (nfs4 goes further and says it must change). 

current code allocates a new, blank, verifier on each write/commit.
since it should never change its better to allocate it once and reuse the same instance.