libbpf: preserve errno across pr_warn/pr_info/pr_debug

As suggested in [0], make sure that libbpf_print saves and restored
errno and as such guaranteed that no matter what actual print callback
user installs, macros like pr_warn/pr_info/pr_debug are completely
transparent as far as errno goes.

While libbpf code is pretty careful about not clobbering important errno
values accidentally with pr_warn(), it's a trivial change to make sure
that pr_warn can be used anywhere without a risk of clobbering errno.

No functional changes, just future proofing.

  [0] https://github.com/libbpf/libbpf/pull/536

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Daniel Müller <deso@posteo.net>
Link: https://lore.kernel.org/r/20220810183425.1998735-1-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/lib/bpf/libbpf.c

@@ -223,13 +223,18 @@ __printf(2, 3)
 void libbpf_print(enum libbpf_print_level level, const char *format, ...)
 {
 	va_list args;
+	int old_errno;
 
 	if (!__libbpf_pr)
 		return;
 
+	old_errno = errno;
+
 	va_start(args, format);
 	__libbpf_pr(level, format, args);
 	va_end(args);
+
+	errno = old_errno;
 }
 
 static void pr_perm_msg(int err)