Add a new ALU opcode, to compute a modulus.

Commit ffe06c17 used an ancillary to implement XOR_X,
but here we reserve one of the available ALU opcode to implement both
MOD_X and MOD_K
Signed-off-by: NEric Dumazet <edumazet@google.com>
Suggested-by: NGeorge Bakos <gbakos@alpinista.org>
Cc: Jay Schulist <jschlst@samba.org>
Cc: Jiri Pirko <jpirko@redhat.com>
Cc: Andi Kleen <ak@linux.intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>