fuse: add more control over cache invalidation behaviour

Currently userspace is able to notify the kernel to invalidate the cache
for an inode.  This means that, if all the inodes in a filesystem need to
be invalidated, then userspace needs to iterate through all of them and do
this kernel notification separately.

This patch adds the concept of 'epoch': each fuse connection will have the
current epoch initialized and every new dentry will have it's d_time set to
the current epoch value.  A new operation will then allow userspace to
increment the epoch value.  Every time a dentry is d_revalidate()'ed, it's
epoch is compared with the current connection epoch and invalidated if it's
value is different.

Signed-off-by: Luis Henriques <luis@igalia.com>
Tested-by: Laura Promberger <laura.promberger@cern.ch>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

include/uapi/linux/fuse.h

@@ -232,6 +232,9 @@
  *
  *  7.43
  *  - add FUSE_REQUEST_TIMEOUT
+ *
+ *  7.44
+ *  - add FUSE_NOTIFY_INC_EPOCH
  */
 
 #ifndef _LINUX_FUSE_H
@@ -267,7 +270,7 @@
 #define FUSE_KERNEL_VERSION 7
 
 /** Minor version number of this interface */
-#define FUSE_KERNEL_MINOR_VERSION 43
+#define FUSE_KERNEL_MINOR_VERSION 44
 
 /** The node ID of the root inode */
 #define FUSE_ROOT_ID 1
@@ -671,6 +674,7 @@ enum fuse_notify_code {
 	FUSE_NOTIFY_RETRIEVE = 5,
 	FUSE_NOTIFY_DELETE = 6,
 	FUSE_NOTIFY_RESEND = 7,
+	FUSE_NOTIFY_INC_EPOCH = 8,
 	FUSE_NOTIFY_CODE_MAX,
 };