fuse: umount should wait for all requests

fuse_abort_conn() does not guarantee that all async requests have actually
finished aborting (i.e. their ->end() function is called).  This could
actually result in still used inodes after umount.

Add a helper to wait until all requests are fully done.  This is done by
looking at the "num_waiting" counter.  When this counter drops to zero, we
can be sure that no more requests are outstanding.

Fixes: 0d8e84b043 ("fuse: simplify request abort")
Cc: <stable@vger.kernel.org> # v4.2
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/fuse/inode.c

@@ -394,6 +394,8 @@ static void fuse_put_super(struct super_block *sb)
 	fuse_send_destroy(fc);
 
 	fuse_abort_conn(fc, false);
+	fuse_wait_aborted(fc);
+
 	mutex_lock(&fuse_mutex);
 	list_del(&fc->entry);
 	fuse_ctl_remove_conn(fc);