erofs: clean up collection handling routines

 - change return value to int since collection is
   already returned within the collector.
 - better function naming.

Link: https://lore.kernel.org/r/20191008125616.183715-1-gaoxiang25@huawei.com


Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>