NFS: Move the write verifier into the nfs_pgio_header

The header had a pointer to the verifier that was set from the old write
data struct.  We don't need to keep the pointer around now that we have
shared structures.

Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>