NFS: Remove unused argument from nfs_direct_write_complete()

This parameter hasn't been used since 2a009ec9 (Linux 3.13-rc3), so
let's remove it from this function.

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