[RFC] xfrm: convert alg_key to flexible array member

[Stephen Hemminger <stephen@networkplumber.org>]

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>
---
 include/uapi/linux/xfrm.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/uapi/linux/xfrm.h b/include/uapi/linux/xfrm.h
index 65e13a099b1a..3ed61df9cc91 100644
--- a/include/uapi/linux/xfrm.h
+++ b/include/uapi/linux/xfrm.h
@@ -102,21 +102,21 @@ struct xfrm_replay_state_esn {
 struct xfrm_algo {
 	char		alg_name[64];
 	unsigned int	alg_key_len;    /* in bits */
-	char		alg_key[0];
+	char		alg_key[];
 };
 
 struct xfrm_algo_auth {
 	char		alg_name[64];
 	unsigned int	alg_key_len;    /* in bits */
 	unsigned int	alg_trunc_len;  /* in bits */
-	char		alg_key[0];
+	char		alg_key[];
 };
 
 struct xfrm_algo_aead {
 	char		alg_name[64];
 	unsigned int	alg_key_len;	/* in bits */
 	unsigned int	alg_icv_len;	/* in bits */
-	char		alg_key[0];
+	char		alg_key[];
 };
 
 struct xfrm_stats {
-- 
2.35.1