erofs: rename per-CPU buffers to global buffer pool and make it configurable

It will cost more time if compressed buffers are allocated on demand for
low-latency algorithms (like lz4) so EROFS uses per-CPU buffers to keep
compressed data if in-place decompression is unfulfilled.  While it is kind
of wasteful of memory for a device with hundreds of CPUs, and only a small
number of CPUs concurrently decompress most of the time.

This patch renames it as 'global buffer pool' and makes it configurable.
This allows two or more CPUs to share a common buffer to reduce memory
occupation.

Suggested-by: Gao Xiang <xiang@kernel.org>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Signed-off-by: Chunhai Guo <guochunhai@vivo.com>
Link: https://lore.kernel.org/r/20240402100036.2673604-1-guochunhai@vivo.com
Signed-off-by: Sandeep Dhavale <dhavale@google.com>
Link: https://lore.kernel.org/r/20240408215231.3376659-1-dhavale@google.com
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>