fuse: fix fsync on directory

Commit ab2257e9941b ("fuse: reduce size of struct fuse_inode") moved parts
of fields related to writeback on regular file and to directory caching
into a union.  However fuse_fsync_common() called from fuse_dir_fsync()
touches some writeback related fields, resulting in a crash.

Move writeback related parts from fuse_fsync_common() to fuse_fysnc().

Reported-by: Brett Girton <btgirton@gmail.com>
Tested-by: Brett Girton <btgirton@gmail.com>
Fixes: ab2257e9941b ("fuse: reduce size of struct fuse_inode")
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
diff --git a/fs/fuse/fuse_i.h b/fs/fuse/fuse_i.h
index e9f712e..afe1f23 100644
--- a/fs/fuse/fuse_i.h
+++ b/fs/fuse/fuse_i.h
@@ -828,7 +828,7 @@ void fuse_release_common(struct file *file, int opcode);
  * Send FSYNC or FSYNCDIR request
  */
 int fuse_fsync_common(struct file *file, loff_t start, loff_t end,
-		      int datasync, int isdir);
+		      int datasync, int opcode);
 
 /**
  * Notify poll wakeup