erofs: get rid of erofs_{find,insert}_workgroup

Just fold them into the only two callers since
they are simple enough.

Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20241021035323.3280682-1-hsiangkao@linux.alibaba.com