erofs: introduce partial-referenced pclusters

Due to deduplication for compressed data, pclusters can be partially
referenced with their prefixes.

Together with the user-space implementation, it enables EROFS
variable-length global compressed data deduplication with rolling
hash.

Link: https://lore.kernel.org/r/20220923014915.4362-1-hsiangkao@linux.alibaba.com


Reviewed-by: Yue Hu <huyue2@coolpad.com>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>