erofs: tidy up zdata.c

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.

Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20250114034429.431408-3-hsiangkao@linux.alibaba.com