fanotify: split version into version and metadata_len

To implement per event type optional headers we are interested in
knowing how long the metadata structure is.  This patch slits the __u32
version field into a __u8 version and a __u16 metadata_len field (with
__u8 left over).  This should allow for backwards compat ABI.

Signed-off-by: Alexey Zaytsev <alexey.zaytsev@gmail.com>
[rewrote descrtion and changed object sizes and ordering - eparis]
Signed-off-by: Eric Paris <eparis@redhat.com>

include/linux/fanotify.h

@@ -83,11 +83,13 @@
 				 FAN_ALL_PERM_EVENTS |\
 				 FAN_Q_OVERFLOW)
 
-#define FANOTIFY_METADATA_VERSION	2
+#define FANOTIFY_METADATA_VERSION	3
 
 struct fanotify_event_metadata {
 	__u32 event_len;
-	__u32 vers;
+	__u8 vers;
+	__u8 reserved;
+	__u16 metadata_len;
 	__aligned_u64 mask;
 	__s32 fd;
 	__s32 pid;