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/fuse_i.h

@@ -862,6 +862,7 @@ void fuse_request_send_background_locked(struct fuse_conn *fc,
 
 /* Abort all requests */
 void fuse_abort_conn(struct fuse_conn *fc, bool is_abort);
+void fuse_wait_aborted(struct fuse_conn *fc);
 
 /**
  * Invalidate inode attributes