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``.

Fixes: ab749bad ("erofs: support unaligned data decompression")
Fixes: cecf864d

 ("erofs: support inline data decompression")
Reviewed-by: Yue Hu <huyue2@coolpad.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Link: https://lore.kernel.org/r/20220512115833.24175-1-hsiangkao@linux.alibaba.com
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>