kernel: support the case that init_task.mnt_ns != zygote.mnt_ns(WSA) (#698)

Basic support for the case that init_task.mnt_ns != zygote.mnt_ns(WSA),
just copy nsproxy and fs pointers for solve #276.

Note the copy in `apk_sign.c` is not required but suggested for
secure(ensure the checked mnt_ns is what ns android running, not created
by user, although many distributions does not have user ns.).

Tested with latest release on Win10 19045.3086(with WSAPatch).

Further review required for:
- [x] Security of this operation (without locking).
- [x] The impact of these modifications on other Android distributions.

kernel/apk_sign.c

@@ -16,7 +16,7 @@ check_v2_signature(char *path, unsigned expected_size, unsigned expected_hash)
 
 	int sign = -1;
 	int i;
-	struct file *fp = filp_open(path, O_RDONLY, 0);
+	struct file *fp = ksu_filp_open_compat(path, O_RDONLY, 0);
 	if (IS_ERR(fp)) {
 		pr_err("open %s error.", path);
 		return PTR_ERR(fp);