Currently, z_erofs_map_blocks_iter() returns whether extents are
compressed or not, and the decompression frontend gets the specific
algorithms then.

It works but not quite well in many aspests, for example:
 - The decompression frontend has to deal with whether extents are
   compressed or not again and lookup the algorithms if compressed.
   It's duplicated and too detailed about the on-disk mapping.

 - A new secondary compression head will be introduced later so that
   each file can have 2 compression algorithms at most for different
   type of data. It could increase the complexity of the decompression
   frontend if still handled in this way;

 - A new readmore decompression strategy will be introduced to get
   better performance for much bigger pcluster and lzma, which needs
   the specific algorithm in advance as well.

Let's look up compression algorithms in z_erofs_map_blocks_iter()
directly instead.

Link: https://lore.kernel.org/r/20211008200839.24541-2-xiang@kernel.orgReviewed-by: Chao Yu <chao@kernel.org>
Reviewed-by: Yue Hu <huyue2@yulong.com>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>