lib/cmdline: allow NULL to be an output for get_option()

In the future we would like to use get_option() to only validate the
string and parse it separately.  To achieve this, allow NULL to be an
output for get_option().

Link: https://lkml.kernel.org/r/20201112180732.75589-5-andriy.shevchenko@linux.intel.com
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Brendan Higgins <brendanhiggins@google.com>
Cc: David Gow <davidgow@google.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Cc: Shuah Khan <skhan@linuxfoundation.org>
Cc: Vitor Massaru Iha <vitor@massaru.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

lib/cmdline.c

@@ -35,11 +35,14 @@ static int get_range(char **str, int *pint, int n)
 /**
  *	get_option - Parse integer from an option string
  *	@str: option string
- *	@pint: (output) integer value parsed from @str
+ *	@pint: (optional output) integer value parsed from @str
  *
  *	Read an int from an option string; if available accept a subsequent
  *	comma as well.
  *
+ *	When @pint is NULL the function can be used as a validator of
+ *	the current option in the string.
+ *
  *	Return values:
  *	0 - no int in string
  *	1 - int found, no subsequent comma
@@ -53,13 +56,16 @@ static int get_range(char **str, int *pint, int n)
 int get_option(char **str, int *pint)
 {
 	char *cur = *str;
+	int value;
 
 	if (!cur || !(*cur))
 		return 0;
 	if (*cur == '-')
-		*pint = -simple_strtoull(++cur, str, 0);
+		value = -simple_strtoull(++cur, str, 0);
 	else
-		*pint = simple_strtoull(cur, str, 0);
+		value = simple_strtoull(cur, str, 0);
+	if (pint)
+		*pint = value;
 	if (cur == *str)
 		return 0;
 	if (**str == ',') {