staging: erofs: move per-CPU buffers implementation to utils.c

This patch moves per-CPU buffers to utils.c in order for
the upcoming generic decompression framework to use it.

Note that I tried to use generic per-CPU buffer or
per-CPU page approaches to clean up further, but obvious
performanace regression (about 2% for sequential read) was
observed.

Therefore let's leave it as it is instead, just move
to utils.c and I'll try to dig into the root cause later.

Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>