net: nfs: don't fail when nfs_read_reply returns -NFS_RPC_DROP

That can happen if duplicate UDP packet arrived, and that's not uncommon.
Anyway, we ignore packets with rpc_id lower than last we sent for other
requests, so it makes sense to do that for read request as well.

Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>