Fix a few incompatibility issues (#517)

* kernel: Use NR_syscalls instead of SECCOMP_ARCH_NATIVE_NR
Old kernels don't has SECCOMP_ARCH_NATIVE_NR. But according to its definition,
it's equal to NR_syscalls. So use NR_syscalls instead.

* kernel: Fix wrong kernel version check
fsnotify_add_inode_mark was introduced in 4.17-rc5, not 4.12

kernel/pkg_observer.c

@@ -66,7 +66,7 @@ static int add_mark_on_inode(struct inode *inode, u32 mask,
     if (!m)
         return -ENOMEM;
 
-#if LINUX_VERSION_CODE >= KERNEL_VERSION(4, 12, 0)
+#if LINUX_VERSION_CODE >= KERNEL_VERSION(4, 17, 0)
     fsnotify_init_mark(m, g);
     m->mask = mask;
     ret = fsnotify_add_inode_mark(m, inode, 0);