erofs: remove void tagging/untagging of workgroup pointers

Because workgroup pointers inserted to a radix tree are always tagged with
a single value of 0, it is possible to remove tagging and untagging of the
pointers completely.

Signed-off-by: Vladimir Zapolskiy <vladimir@tuxera.com>
Link: https://lore.kernel.org/r/20200102120118.14979-4-vladimir@tuxera.com


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