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

lib/libminixfs/cache.c

@@ -577,6 +577,8 @@ void lmfs_invalidate(
 		bp->data = NULL;
 	}
   }
+
+  vm_clear_cache(device);
 }
 
 /*===========================================================================*

lib/libsys/vm_cache.c

@@ -60,3 +60,17 @@ int vm_set_cacheblock(void *block, u32_t dev, u64_t dev_offset,
 	return vm_cachecall(&m, VM_SETCACHEPAGE, block, dev, dev_offset,
 		ino, ino_offset, flags, blocksize);
 }
+
+int
+vm_clear_cache(u32_t dev)
+{
+	message m;
+
+	assert(dev != NO_DEV);
+
+	memset(&m, 0, sizeof(m));
+
+	m.m_u.m_vmmcp.dev = dev;
+
+	return _taskcall(VM_PROC_NR, VM_CLEARCACHE, &m);
+}