fuse: use iversion for readdir cache verification

Use the internal iversion counter to make sure modifications of the
directory through this filesystem are not missed by the mtime check (due to
mtime granularity).

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/fuse/fuse_i.h

@@ -120,6 +120,9 @@ struct fuse_inode {
 		/* modification time of directory when cache was started */
 		struct timespec64 mtime;
 
+		/* iversion of directory when cache was started */
+		u64 iversion;
+
 		/* protects above fields */
 		spinlock_t lock;
 	} rdc;