erofs: remove unneeded parameter

After commit 0615090c ("erofs: convert compressed files from
readpages to readahead"), add_to_page_cache_lru() was moved to mm
code, so that in below call path, no page will be cached into
@pagepool list or grabbed from @pagepool list:
- z_erofs_readpage
 - z_erofs_do_read_page
  - preload_compressed_pages
  - erofs_allocpage

Let's get rid of this unneeded @pagepool parameter.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Link: https://lore.kernel.org/r/20200917011821.22767-1-yuchao0@huawei.com


Reviewed-by: Gao Xiang <hsiangkao@redhat.com>
Signed-off-by: Gao Xiang <hsiangkao@redhat.com>