string: factorize skip_spaces and export it to be generally available

On the following sentence:
while (*s && isspace(*s))
s++;

If *s == 0, isspace() evaluates to ((_ctype[*s] & 0x20) != 0), which
evaluates to ((0x08 & 0x20) != 0) which equals to 0 as well.
If *s == 1, we depend on isspace() result anyway. In other words,
"a char equals zero is never a space", so remove this check.

Also, *s != 0 is most common case (non-null string).

Fixed const return as noticed by Jan Engelhardt and James Bottomley.
Fixed unnecessary extra cast on strstrip() as noticed by Jan Engelhardt.

Signed-off-by: André Goddard Rosa <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

include/linux/ctype.h

···
 #define islower(c)	((__ismask(c)&(_L)) != 0)
 #define isprint(c)	((__ismask(c)&(_P|_U|_L|_D|_SP)) != 0)
 #define ispunct(c)	((__ismask(c)&(_P)) != 0)
+/* Note: isspace() must return false for %NUL-terminator */
 #define isspace(c)	((__ismask(c)&(_S)) != 0)
 #define isupper(c)	((__ismask(c)&(_U)) != 0)
 #define isxdigit(c)	((__ismask(c)&(_D|_X)) != 0)

include/linux/string.h

···
 #ifndef __HAVE_ARCH_STRRCHR
 extern char * strrchr(const char *,int);
 #endif
+extern char * __must_check skip_spaces(const char *);
 extern char * __must_check strstrip(char *);
 #ifndef __HAVE_ARCH_STRSTR
 extern char * strstr(const char *,const char *);

lib/string.c

···
 #endif
 
 /**
+ * skip_spaces - Removes leading whitespace from @s.
+ * @s: The string to be stripped.
+ *
+ * Returns a pointer to the first non-whitespace character in @s.
+ */
+char *skip_spaces(const char *str)
+{
+	while (isspace(*str))
+		++str;
+	return (char *)str;
+}
+EXPORT_SYMBOL(skip_spaces);
+
+/**
  * strstrip - Removes leading and trailing whitespace from @s.
  * @s: The string to be stripped.
  *
···
 		end--;
 	*(end + 1) = '\0';
 
-	while (*s && isspace(*s))
-		s++;
-
-	return s;
+	return skip_spaces(s);
 }
 EXPORT_SYMBOL(strstrip);