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

include/minix/com.h

@@ -978,6 +978,9 @@
 /* To VM: identify cache block in FS */
 #define VM_SETCACHEPAGE		(VM_RQ_BASE+27)
 
+/* To VM: clear all cache blocks for a device */
+#define VM_CLEARCACHE		(VM_RQ_BASE+28)
+
 /* To VFS: fields for request from VM. */
 #	define VFS_VMCALL_REQ		m10_i1
 #	define VFS_VMCALL_FD		m10_i2

include/minix/vm.h

@@ -72,6 +72,8 @@ int vm_set_cacheblock(void *block, u32_t dev, u64_t dev_offset,
 void *vm_map_cacheblock(u32_t dev, u64_t dev_offset,
         u64_t ino, u64_t ino_offset, u32_t *flags, int blocksize);
 
+int vm_clear_cache(u32_t dev);
+
 /* flags for vm cache functions */
 #define VMMC_FLAGS_LOCKED	0x01	/* someone is updating the flags; don't read/write */
 #define VMMC_DIRTY		0x02	/* dirty buffer and it may not be evicted */