include/linux/ctype.h: make isdigit() table lookupless

Make isdigit into a simple range checking inline function:

	return '0' <= c && c <= '9';

This code is 1 branch, not 2 because any reasonable compiler can
optimize this code into SUB+CMP, so the code

	while (isdigit((c = *s++)))
		...

remains 1 branch per iteration HOWEVER it suddenly doesn't do table
lookup priming cacheline nobody cares about.

Link: http://lkml.kernel.org/r/20160826190047.GA12536@p183.telecom.by
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/ctype.h

@@ -22,7 +22,10 @@ extern const unsigned char _ctype[];
 #define isalnum(c)	((__ismask(c)&(_U|_L|_D)) != 0)
 #define isalpha(c)	((__ismask(c)&(_U|_L)) != 0)
 #define iscntrl(c)	((__ismask(c)&(_C)) != 0)
-#define isdigit(c)	((__ismask(c)&(_D)) != 0)
+static inline int isdigit(int c)
+{
+	return '0' <= c && c <= '9';
+}
 #define isgraph(c)	((__ismask(c)&(_P|_U|_L|_D)) != 0)
 #define islower(c)	((__ismask(c)&(_L)) != 0)
 #define isprint(c)	((__ismask(c)&(_P|_U|_L|_D|_SP)) != 0)