erofs: rename some generic methods in decompressor

Previously, some LZ4 methods were named with `generic'. However, while
evaluating the effective LZMA approach, it seems they aren't quite
generic at all (e.g. no need preparing dstpages for most LZMA cases.)

Avoid such naming instead.

Link: https://lore.kernel.org/r/20211010213145.17462-7-xiang@kernel.org


Acked-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>