erofs: introduce `z_erofs_parse_in_bvecs'

`z_erofs_decompress_pcluster()' is too long therefore it'd be better
to introduce another helper to parse compressed pages (or laterly,
compressed bvecs.)

BTW, since `compressed_bvecs' is too long as a part of the function
name, `in_bvecs' is used here instead.

Acked-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20220715154203.48093-7-hsiangkao@linux.alibaba.com