erofs: clean up z_erofs_collector_begin()

Rearrange the code and get rid of all gotos.

Reviewed-by: Yue Hu <huyue2@coolpad.com>
Acked-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20220715154203.48093-3-hsiangkao@linux.alibaba.com