VM: readd support for forgetting cached FS blocks

Not all services involved in block I/O go through VM to access the
blocks they need.  As a result, the blocks in VM may become stale,
possibly causing corruption when the stale copy is restored by a
service that does go through VM later on.  This patch restores support
for forgetting cached blocks that belong to a particular device, and
makes the relevant file systems use this functionality 1) when
requested by VFS through REQ_FLUSH, and 2) upon unmount.

Change-Id: I0758c5ed8fe4b5ba81d432595d2113175776aff8

test/test72.c

@@ -224,16 +224,6 @@ panic(const char *fmt, ...)
 	exit(1);
 }
 
-int vm_forgetblock(u64_t id)
-{
-	return ENOSYS;
-}
-
-void vm_forgetblocks(void)
-{
-	return;
-}
-
 int
 vm_info_stats(struct vm_stats_info *vsi)
 {
@@ -274,6 +264,11 @@ void *vm_map_cacheblock(u32_t dev, u64_t dev_offset,
 	return MAP_FAILED;
 }
 
+int vm_clear_cache(u32_t dev)
+{
+	return 0;
+}
+
 int
 main(int argc, char *argv[])
 {

test/testvm.c

@@ -177,6 +177,8 @@ main(int argc, char *argv[])
 
 	writepipe(&info);
 
+	vm_clear_cache(MYDEV);
+
 	return 0;
 }
 

test/testvm.conf

@@ -1,7 +1,7 @@
 service testvm {
 	ipc	ALL;	# All system ipc targets allowed
 	system	BASIC;		# Only basic kernel calls allowed
-	vm      MAPCACHEPAGE SETCACHEPAGE;
+	vm      MAPCACHEPAGE SETCACHEPAGE CLEARCACHE;
 	io	NONE;		# No I/O range allowed
 	irq	NONE;		# No IRQ allowed
 	sigmgr          rs;	# Signal manager is RS