vsprintf: Use __diag macros to disable '-Wsuggest-attribute=format'

The GCC specific warning '-Wsuggest-attribute=format' is disabled around
va_format() using raw #pragma statements, which includes an
'#ifndef __clang__' to avoid a warning about an unknown warning option
from clang (which recognizes '#pragma GCC' for compatibility reasons):

  lib/vsprintf.c:1703:32: error: unknown warning group '-Wsuggest-attribute=format', ignored [-Werror,-Wunknown-warning-option]
   1703 | #pragma GCC diagnostic ignored "-Wsuggest-attribute=format"
        |                                ^

While the current solution works, it is not visually appealing. The
kernel already has some infrastructure that wraps these #pragma
statements to give more specific control over diagnostics without
needing #ifdef blocks for different compilers. Convert the existing
statements over to the __diag macros.

Closes: https://lore.kernel.org/r/CAHk-=wgfX9nBGE0Ap9GjhOy7Mn=RSy=rx0MvqfYFFDx31KJXqQ@mail.gmail.com
Signed-off-by: Nathan Chancellor <[email protected]>
Tested-by: Andy Shevchenko <[email protected]>
Reviewed-by: Petr Mladek <[email protected]>
Link: https://patch.msgid.link/20250404-vsprintf-convert-pragmas-to-__diag-v1-2-5d6c5c55b2bd@kernel.org
Signed-off-by: Petr Mladek <[email protected]>

Author: nathanchance

lib/vsprintf.c

@@ -1699,10 +1699,9 @@ char *escaped_string(char *buf, char *end, u8 *addr, struct printf_spec spec,
 	return buf;
 }
 
-#pragma GCC diagnostic push
-#ifndef __clang__
-#pragma GCC diagnostic ignored "-Wsuggest-attribute=format"
-#endif
+__diag_push();
+__diag_ignore(GCC, all, "-Wsuggest-attribute=format",
+	      "Not a valid __printf() conversion candidate.");
 static char *va_format(char *buf, char *end, struct va_format *va_fmt,
 		       struct printf_spec spec)
 {
@@ -1717,7 +1716,7 @@ static char *va_format(char *buf, char *end, struct va_format *va_fmt,
 
 	return buf;
 }
-#pragma GCC diagnostic pop
+__diag_pop();
 
 static noinline_for_stack
 char *uuid_string(char *buf, char *end, const u8 *addr,