m68k: Remove inline strlen() implementation

GCC can replace a strncat() call with constant second argument into a
strlen + store, which results in a link error:

ERROR: "strlen" [net/ipv4/ip_tunnel.ko] undefined!

The inline function is a simple for loop in C. Other architectures
either use an asm optimized variant, or use the generic function from
lib/string.c.
Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>

arch/m68k/include/asm/string.h

@@ -4,15 +4,6 @@
 #include <linux/types.h>
 #include <linux/compiler.h>
 
-static inline size_t __kernel_strlen(const char *s)
-{
-	const char *sc;
-
-	for (sc = s; *sc++; )
-		;
-	return sc - s - 1;
-}
-
 static inline char *__kernel_strcpy(char *dest, const char *src)
 {
 	char *xdest = dest;
@@ -27,11 +18,6 @@ static inline char *__kernel_strcpy(char *dest, const char *src)
 
 #ifndef __IN_STRING_C
 
-#define __HAVE_ARCH_STRLEN
-#define strlen(s)	(__builtin_constant_p(s) ?	\
-			 __builtin_strlen(s) :		\
-			 __kernel_strlen(s))
-
 #define __HAVE_ARCH_STRNLEN
 static inline size_t strnlen(const char *s, size_t count)
 {

arch/m68k/lib/string.c

@@ -17,6 +17,6 @@ EXPORT_SYMBOL(strcpy);
 
 char *strcat(char *dest, const char *src)
 {
-	return __kernel_strcpy(dest + __kernel_strlen(dest), src);
+	return __kernel_strcpy(dest + strlen(dest), src);
 }
 EXPORT_SYMBOL(strcat);