NFS: Fix an incorrect type in struct nfs_direct_req

The start offset needs to be of type loff_t.

Fixed: 5fadeb47 ("nfs: count DIO good bytes correctly with mirroring")
Cc: stable@vger.kernel.org # v4.0+
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>