kernel: backport to 4.4 (#166)

These changes make KernelSU work on kernel4.4
[link](https://github.com/F-19-F/android_kernel_oneplus_msm8998).
LINUX_VERSION_CODE macro changes have been vertied on 4.4 4.9 4.14.
For kernel 4.4,just pick two commits
* [introduce
KernelSU](2993524f2f)
* [allow init exec ksud under
nosuid](3df9df42a6)

kernel/kernel_compat.h

@@ -0,0 +1,3 @@
+#include "linux/fs.h"
+extern ssize_t kernel_read_compat(struct file *p, void* buf, size_t count, loff_t *pos);
+extern ssize_t kernel_write_compat(struct file *p, const void *buf, size_t count, loff_t *pos);