erofs: record the longest decompressed size in this round

Currently, `pcl->length' records the longest decompressed length
as long as the pcluster itself isn't reclaimed.  However, such
number is unneeded for the general cases since it doesn't indicate
the exact decompressed size in this round.

Instead, let's record the decompressed size for this round instead,
thus `pcl->nr_pages' can be completely dropped and pageofs_out is
also designed to be kept in sync with `pcl->length'.

Acked-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20220715154203.48093-16-hsiangkao@linux.alibaba.com