xfrm: convert alg_key to flexible array member

Iproute2 build generates a warning when built with gcc-12.
This is because the alg_key in xfrm.h API has zero size
array element instead of flexible array.

    CC       xfrm_state.o
In function ‘xfrm_algo_parse’,
    inlined from ‘xfrm_state_modify.constprop’ at xfrm_state.c:573:5:
xfrm_state.c:162:32: warning: writing 1 byte into a region of size 0 [-Wstringop-overflow=]
  162 |                         buf[j] = val;
      |                         ~~~~~~~^~~~~

This patch convert the alg_key into flexible array member.
There are other zero size arrays here that should be converted as
well.

This patch is RFC only since it is only compile tested and
passes trivial iproute2 tests.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>