functionfs: need to cancel ->reset_work in ->kill_sb()

... otherwise we just might free ffs with ffs->reset_work
still on queue.  That needs to be done after ffs_data_reset() -
that's the cutoff point for configfs accesses (serialized
on gadget_info->lock), which is where the schedule_work()
would come from.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

drivers/usb/gadget/function/f_fs.c

@@ -2081,6 +2081,9 @@ ffs_fs_kill_sb(struct super_block *sb)
 		struct ffs_data *ffs = sb->s_fs_info;
 		ffs->state = FFS_CLOSING;
 		ffs_data_reset(ffs);
+		// no configfs accesses from that point on,
+		// so no further schedule_work() is possible
+		cancel_work_sync(&ffs->reset_work);
 		ffs_data_put(ffs);
 	}
 }