selftests/bpf: fix constness of source arg for bpf helpers

Fix signature of bpf_probe_read and bpf_probe_write_user to mark source
pointer as const. This causes warnings during compilation for
applications relying on those helpers.
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/bpf_helpers.h

@@ -31,7 +31,7 @@ static int (*bpf_map_pop_elem)(void *map, void *value) =
 	(void *) BPF_FUNC_map_pop_elem;
 static int (*bpf_map_peek_elem)(void *map, void *value) =
 	(void *) BPF_FUNC_map_peek_elem;
-static int (*bpf_probe_read)(void *dst, int size, void *unsafe_ptr) =
+static int (*bpf_probe_read)(void *dst, int size, const void *unsafe_ptr) =
 	(void *) BPF_FUNC_probe_read;
 static unsigned long long (*bpf_ktime_get_ns)(void) =
 	(void *) BPF_FUNC_ktime_get_ns;
@@ -62,7 +62,7 @@ static int (*bpf_perf_event_output)(void *ctx, void *map,
 	(void *) BPF_FUNC_perf_event_output;
 static int (*bpf_get_stackid)(void *ctx, void *map, int flags) =
 	(void *) BPF_FUNC_get_stackid;
-static int (*bpf_probe_write_user)(void *dst, void *src, int size) =
+static int (*bpf_probe_write_user)(void *dst, const void *src, int size) =
 	(void *) BPF_FUNC_probe_write_user;
 static int (*bpf_current_task_under_cgroup)(void *map, int index) =
 	(void *) BPF_FUNC_current_task_under_cgroup;