erofs: get rid of `z_pagemap_global'

In order to introduce multi-reference pclusters for compressed data
deduplication, let's get rid of the global page array for now since
it needs to be re-designed then at least.

Acked-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20220715154203.48093-12-hsiangkao@linux.alibaba.com