netfilter: implement xt_cgroup cgroup2 path match

netfilter: implement xt_cgroup cgroup2 path match

This patch implements xt_cgroup path match which matches cgroup2
membership of the associated socket.  The match is recursive and
invertible.

For rationales on introducing another cgroup based match, please refer
to a preceding commit "sock, cgroup: add sock->sk_cgroup".

v3: Folded into xt_cgroup as a new revision interface as suggested by
    Pablo.

v2: Included linux/limits.h from xt_cgroup2.h for PATH_MAX.  Added
    explicit alignment to the priv field.  Both suggested by Jan.

Signed-off-by: Tejun Heo <[email protected]>
Cc: Daniel Borkmann <[email protected]>
Cc: Daniel Wagner <[email protected]>
CC: Neil Horman <[email protected]>
Cc: Jan Engelhardt <[email protected]>
Cc: Pablo Neira Ayuso <[email protected]>
Signed-off-by: Pablo Neira Ayuso <[email protected]>