bpf: Fix bpf_strncmp proto.

bpf_strncmp() doesn't write into its first argument.
Make sure that the verifier knows about it.
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20230313235845.61029-2-alexei.starovoitov@gmail.comSigned-off-by: Martin KaFai Lau <martin.lau@kernel.org>

kernel/bpf/helpers.c

@@ -571,7 +571,7 @@ static const struct bpf_func_proto bpf_strncmp_proto = {
 	.func		= bpf_strncmp,
 	.gpl_only	= false,
 	.ret_type	= RET_INTEGER,
-	.arg1_type	= ARG_PTR_TO_MEM,
+	.arg1_type	= ARG_PTR_TO_MEM | MEM_RDONLY,
 	.arg2_type	= ARG_CONST_SIZE,
 	.arg3_type	= ARG_PTR_TO_CONST_STR,
 };