erofs: introduce `z_erofs_parse_out_bvecs()'

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

BTW, since `decompressed_bvecs' is too long as a part of the function
name, `out_bvecs' is used instead.

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/20220715154203.48093-4-hsiangkao@linux.alibaba.com