inet: move ir_mark to fill a hole

On 64bit arches, we can save 8 bytes in inet_request_sock
by moving ir_mark to fill a hole.

While we are at it, inet_request_mark() can get a const qualifier
for listener socket.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>