All small code style adjustments, no logic changes:
- z_erofs_decompress_frontend => z_erofs_frontend;
- z_erofs_decompress_backend => z_erofs_backend;
- Use Z_EROFS_DEFINE_FRONTEND() to replace DECOMPRESS_FRONTEND_INIT();
- `nr_folios` should be `nrpages` in z_erofs_readahead();
- Refine in-line comments.
Signed-off-by: Gao Xiang <hsiangkao@xxxxxxxxxxxxxxxxx>