erofs: move {in,out}pages into struct z_erofs_decompress_req

It seems that all compressors need those two values, so just move
them into the common structure.

`struct z_erofs_lz4_decompress_ctx` can be dropped too.

Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20250305124007.1810731-1-hsiangkao@linux.alibaba.com