uapi headers: simplify the <linux/compiler.h> stuff.

We don't really need <linux/compiler.h> and <linux/compiler_types.h>. We
already have a mechanism to remove unused macros, so let's do that. We
don't currently have a way to remove unused #includes, so we still need
<linux/compiler.h> and <linux/compiler_types.h> files (but I've clarified
the comments in them).

I've kept the empty definitions of `__user` and `__force` for source
compatibility. (We had one security test at least that was assuming
a kernel struct definition will "just work".)

Bug: http://b/262917450
Test: treehugger
Change-Id: Iacbbbc1aeef9a4fac52dabd7811ab875cc267d4f
diff --git a/libc/kernel/uapi/linux/dlm_device.h b/libc/kernel/uapi/linux/dlm_device.h
index 5e54d2e..e1b0c6d 100644
--- a/libc/kernel/uapi/linux/dlm_device.h
+++ b/libc/kernel/uapi/linux/dlm_device.h
@@ -33,11 +33,11 @@
   __u32 parent;
   __u64 xid;
   __u64 timeout;
-  void __user * castparam;
-  void __user * castaddr;
-  void __user * bastparam;
-  void __user * bastaddr;
-  struct dlm_lksb __user * lksb;
+  void  * castparam;
+  void  * castaddr;
+  void  * bastparam;
+  void  * bastaddr;
+  struct dlm_lksb  * lksb;
   char lvb[DLM_USER_LVB_LEN];
   char name[];
 };
@@ -67,9 +67,9 @@
 struct dlm_lock_result {
   __u32 version[3];
   __u32 length;
-  void __user * user_astaddr;
-  void __user * user_astparam;
-  struct dlm_lksb __user * user_lksb;
+  void  * user_astaddr;
+  void  * user_astparam;
+  struct dlm_lksb  * user_lksb;
   struct dlm_lksb lksb;
   __u8 bast_mode;
   __u8 unused[3];