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.com


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>