ila: Create net/ipv6/ila directory

Create ila directory in preparation for supporting other hooks in the
kernel than LWT for doing ILA. This includes:
  - Moving ila.c to ila/ila_lwt.c
  - Splitting out some common functions into ila_common.c

Signed-off-by: Tom Herbert <tom@herbertland.com>
Signed-off-by: David S. Miller <davem@davemloft.net>