erofs: fix buffer copy overflow of ztailpacking feature

In the Linux kernel, the following vulnerability has been resolved:

erofs: fix buffer copy overflow of ztailpacking feature

I got some KASAN report as below:

[ 46.959738] ==================================================================

[ 46.960430] BUG: KASAN: use-after-free in z_erofs_shifted_transform+0x2bd/0x370

[ 46.960430] Read of size 4074 at addr ffff8880300c2f8e by task fssum/188

...

[ 46.960430] Call Trace:

[ 46.960430] <TASK>

[ 46.960430] dump_stack_lvl+0x41/0x5e

[ 46.960430] print_report.cold+0xb2/0x6b7

[ 46.960430] ? z_erofs_shifted_transform+0x2bd/0x370

[ 46.960430] kasan_report+0x8a/0x140

[ 46.960430] ? z_erofs_shifted_transform+0x2bd/0x370

[ 46.960430] kasan_check_range+0x14d/0x1d0

[ 46.960430] memcpy+0x20/0x60

[ 46.960430] z_erofs_shifted_transform+0x2bd/0x370

[ 46.960430] z_erofs_decompress_pcluster+0xaae/0x1080

The root cause is that the tail pcluster won't be a complete filesystem

block anymore. So if ztailpacking is used, the second part of an

uncompressed tail pcluster may not be ``rq->pageofs_out``.