erofs: tidy up stream decompressors

Just use a generic helper to prepare buffers for all supported
stream decompressors, eliminating similar logic.

Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20240709094106.3018109-3-hsiangkao@linux.alibaba.com