bpf: Specify access type of bpf_sysctl_get_name args

The second argument of bpf_sysctl_get_name() helper is a pointer to a
buffer that is being written to. However that isn't specify in the
prototype.

Until commit 37cce22d ("bpf: verifier: Refactor helper access
type tracking"), all helper accesses were considered as a possible
write access by the verifier, so no big harm was done. However, since
then, the verifier might make wrong asssumption about the content of
that address which might lead it to make faulty optimizations (such as
removing code that was wrongly labeled dead). This is what happens in
test_sysctl selftest to the tests related to sysctl_get_name.

Add MEM_WRITE flag the second argument of bpf_sysctl_get_name().

Signed-off-by: Jerome Marchand <jmarchan@redhat.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20250619140603.148942-2-jmarchan@redhat.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>