datatype: accept a numeric cgroupsv2 id on input

datatype: accept a numeric cgroupsv2 id on input

nft prints non-existent cgroup names as the raw id using PRIu64, but
cgroupv2_type_parse() only stats /sys/fs/cgroup/<identifier>, so the
listing does not load back:

  # nft list set ip t s
  table ip t {
          set s {
                  type cgroupsv2
                  elements = { 50834 }
          }
  }
  # nft delete element ip t s { 50834 }
  Error: cgroupv2 path fails: No such file or directory

The element cannot be deleted by key once its cgroup is gone, only
flushed with the set, and a dump does not restore. The json dump has
the same problem, and the stale id has been visible in the wild since
2022 (see Link). tests/shell/testcases/packetpath/cgroupv2 already
works around this in cleanup().

Fall back to integer_type_parse() when the path does not resolve, as
boolean_type_parse() already does. The path lookup stays first, so a
cgroup named as a number still resolves as a path. Improving the
integer parser is left for a follow-up as discussed in v1.

Fixes: 38228087252c ("src: add cgroupsv2 support")
Link: https://lore.kernel.org/netfilter-devel/fabde324-383a-622c-7e69-32c9b2d06191@gmail.com/
Signed-off-by: Avinash Duduskar <avinash.duduskar@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>