bpf: fix incorrect initialization of bpf_ctx_convert_map

gcc -Wextra points out that a field may get overridden in some
configurations such as x86 allmodconfig, when the next index after the one
that has been assigned last already had a value, in this case for index
BPF_PROG_TYPE_SK_LOOKUP, which comes after BPF_PROG_TYPE_LSM in the list:

kernel/bpf/btf.c:4225:2: warning: initialized field overwritten [-Woverride-init]
 4225 |  0, /* avoid empty array */
      |  ^
kernel/bpf/btf.c:4225:2: note: (near initialization for 'bpf_ctx_convert_map[30]')

Move the zero-initializer first instead. This avoids the warning since
nothing else uses index 0, and the last element does not have to be zero.

Fixes: e9ddbb7 ("bpf: Introduce SK_LOOKUP program type with a dedicated attach point")
Fixes: 4c80c7b ("bpf: Fix build in minimal configurations, again")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

kernel/bpf/btf.c

@@ -4218,11 +4218,11 @@ enum {
 	__ctx_convert_unused, /* to avoid empty enum in extreme .config */
 };
 static u8 bpf_ctx_convert_map[] = {
+	[0] = 0, /* avoid empty array */
 #define BPF_PROG_TYPE(_id, _name, prog_ctx_type, kern_ctx_type) \
 	[_id] = __ctx_convert##_id,
 #include <linux/bpf_types.h>
 #undef BPF_PROG_TYPE
-	0, /* avoid empty array */
 };
 #undef BPF_MAP_TYPE
 #undef BPF_LINK_TYPE