staging: erofs: introduce pagevec for unzip subsystem

For each compressed cluster, there is a straight-forward
way of allocating a fixed or variable-sized (for VLE) array
to record the corresponding file pages for its decompression
if we decide to decompress these pages asynchronously (eg.
read-ahead case), however it could take much extra on-heap
memory compared with traditional uncompressed filesystems.

This patch introduces a pagevec solution to reuse some
allocated file page in the time-sharing approach storing
parts of the array itself in order to minimize the extra
memory overhead, thus only a constant and small-sized array
used for booting the whole array itself up will be needed.

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