linux/container_of.h: Remove unnecessary cast

Casts are dangerous.
Moreover, (almost) everything converts to 'void *' implicitly,
with the exception of pointers to const.

Change the temporary to be const,
and therefore allow implicit conversion from everything.
Since we don't modify it,
this also adds safety.

Signed-off-by: Alejandro Colomar <alx.manpages@gmail.com>

include/linux/container_of.h

@@ -17,7 +17,7 @@
  *
  */
 #define container_of(ptr, type, member) ({				\
-	void *__mptr = (void *)(ptr);					\
+	const void *__mptr = (ptr);					\
 	static_assert(__same_type(*(ptr), memberof(type, member)) ||	\
 		      __same_type(*(ptr), void),			\
 		      "pointer type mismatch in container_of()");	\