NFS: Fix documenting comment for nfs_create_request()

Clean up: the first parameter of nfs_create_request() has been
incorrectly documented since time immemorial (OK, since before
2.6.12).

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>