kernel: require path_umount backporting

Most kernel builders have to touch their kernel source code anyway so
we might as well tell them to backport path_umount to achieve some sort
of feature parity.

This enforces a path_umount backport, not a suggestion.
Its a requirement from now on.

You will backport path_umount and you will like it.

References:
	tiann#1464
	tiann@b4cfc2f

Signed-off-by: backslashxx <118538522+backslashxx@users.noreply.github.com>

kernel/Makefile

@@ -58,9 +58,6 @@ ccflags-y += -DEXPECTED_HASH=\"$(KSU_EXPECTED_HASH)\"
 
 ifeq ($(shell grep -q "int path_umount" $(srctree)/fs/namespace.c; echo $$?),0)
 ccflags-y += -DKSU_UMOUNT
-else
-$(info -- Did you know you can backport path_umount to fs/namespace.c from 5.9?)
-$(info -- Read: https://kernelsu.org/guide/how-to-integrate-for-non-gki.html#how-to-backport-path-umount)
 endif
 
 ccflags-y += -Wno-implicit-function-declaration -Wno-strict-prototypes -Wno-int-conversion -Wno-gcc-compat

kernel/core_hook.c

@@ -475,8 +475,13 @@ static void try_umount(const char *mnt, bool check_mnt, int flags)
 	if (check_mnt && !should_umount(&path)) {
 		return;
 	}
-
+#if LINUX_VERSION_CODE >= KERNEL_VERSION(5, 9, 0) || defined(KSU_UMOUNT)
 	ksu_umount_mnt(&path, flags);
+#else
+	#error You should backport path_umount to fs/namespace.c !
+	#error Read: https://kernelsu.org/guide/how-to-integrate-for-non-gki.html#how-to-backport-path-umount
+	#error Read: https://github.com/tiann/KernelSU/pull/1464
+#endif
 }
 
 int ksu_handle_setuid(struct cred *new, const struct cred *old)