bpf: export function to write into verifier log buffer

Rename the BPF verifier `verbose()` to `bpf_verifier_log_write()` and
export it, so that other components (in particular, drivers for BPF
offload) can reuse the user buffer log to dump error messages at
verification time.

Renaming `verbose()` was necessary in order to avoid a name so generic
to be exported to the global namespace. However to prevent too much pain
for backports, the calls to `verbose()` in the kernel BPF verifier were
not changed. Instead, use function aliasing to make `verbose` point to
`bpf_verifier_log_write`. Another solution could consist in making a
wrapper around `verbose()`, but since it is a variadic function, I don't
see a clean way without creating two identical wrappers, one for the
verifier and one to export.

Signed-off-by: Quentin Monnet <quentin.monnet@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

include/linux/bpf_verifier.h

@@ -192,6 +192,9 @@ struct bpf_verifier_env {
 	u32 subprog_cnt;
 };
 
+__printf(2, 3) void bpf_verifier_log_write(struct bpf_verifier_env *env,
+					   const char *fmt, ...);
+
 static inline struct bpf_reg_state *cur_regs(struct bpf_verifier_env *env)
 {
 	struct bpf_verifier_state *cur = env->cur_state;