compat: let architectures define __ARCH_WANT_COMPAT_SYS_GETDENTS64

For architecture dependent compat syscalls in common code an architecture
must define something like __ARCH_WANT_<WHATEVER> if it wants to use the
code.
This however is not true for compat_sys_getdents64 for which architectures
must define __ARCH_OMIT_COMPAT_SYS_GETDENTS64 if they do not want the code.

This leads to the situation where all architectures, except mips, get the
compat code but only x86_64, arm64 and the generic syscall architectures
actually use it.

So invert the logic, so that architectures actively must do something to
get the compat code.

This way a couple of architectures get rid of otherwise dead code.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>

fs/compat.c

@@ -981,7 +981,7 @@ asmlinkage long compat_sys_getdents(unsigned int fd,
 	return error;
 }
 
-#ifndef __ARCH_OMIT_COMPAT_SYS_GETDENTS64
+#ifdef __ARCH_WANT_COMPAT_SYS_GETDENTS64
 
 struct compat_getdents_callback64 {
 	struct dir_context ctx;
@@ -1066,7 +1066,7 @@ asmlinkage long compat_sys_getdents64(unsigned int fd,
 	fdput(f);
 	return error;
 }
-#endif /* ! __ARCH_OMIT_COMPAT_SYS_GETDENTS64 */
+#endif /* __ARCH_WANT_COMPAT_SYS_GETDENTS64 */
 
 /*
  * Exactly like fs/open.c:sys_open(), except that it doesn't set the