erofs-utils: lib: fix incorrect nblocks in block list for chunked inodes

erofs-utils: lib: fix incorrect nblocks in block list for chunked inodes

Currently, the number of physical blocks (nblocks) for the last chunk
written to the block list file is incorrectly recorded as the inode
chunksize.

This patch writes the actual number of physical blocks for the inode in
the last chunk to the block list file.

Fixes: 7b46f7a0160a ("erofs-utils: lib: merge consecutive chunks if possible")
Fixes: b6749839e710 ("erofs-utils: generate preallocated extents for tarerofs")
Signed-off-by: Hongzhen Luo <hongzhen@linux.alibaba.com>
Link: https://lore.kernel.org/r/20240911085531.2133723-1-hongzhen@linux.alibaba.com
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>