string: fix prototype of memdup()

It doesn't make sense to restrict memdup() to only return char*
pointers, especially when it is already defined to accept void*. This
makes it uglier to use to e.g. duplicate a struct.

Make it return void*, just as kmemdup() does in the kernel (and which
our kmemdup() in fact also does).

While in here, make a small optimization: memcpy() is defined to
return the destination register, so we write this in a way that the
compiler may do a tail call.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Rasmus Villemoes <ravi@prevas.dk>

include/linux/string.h

@@ -142,7 +142,7 @@ void *memchr_inv(const void *, int, size_t);
  *	memory is available
  *
  */
-char *memdup(const void *src, size_t len);
+void *memdup(const void *src, size_t len);
 
 unsigned long ustrtoul(const char *cp, char **endp, unsigned int base);
 unsigned long long ustrtoull(const char *cp, char **endp, unsigned int base);