staging: erofs: introduce cached decompression

This patch adds an optional choice which can be
enabled by users in order to cache both incomplete
ends of compressed clusters as a complement to
the in-place decompression in order to boost random
read, but it costs more memory than the in-place
decompression only.

Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>