erofs: Fix pcluster memleak when its block address is zero

erofs: Fix pcluster memleak when its block address is zero

syzkaller reported a memleak:
https://syzkaller.appspot.com/bug?id=62f37ff612f0021641eda5b17f056f1668aa9aed

unreferenced object 0xffff88811009c7f8 (size 136):
  ...
  backtrace:
    [<ffffffff821db19b>] z_erofs_do_read_page+0x99b/0x1740
    [<ffffffff821dee9e>] z_erofs_readahead+0x24e/0x580
    [<ffffffff814bc0d6>] read_pages+0x86/0x3d0
    ...

syzkaller constructed a case: in z_erofs_register_pcluster(),
ztailpacking = false and map->m_pa = zero. This makes pcl->obj.index be
zero although pcl is not a inline pcluster.

Then following path adds refcount for grp, but the refcount won't be put
because pcl is inline.

z_erofs_readahead()
  z_erofs_do_read_page() # for another page
    z_erofs_collector_begin()
      erofs_find_workgroup()
        erofs_workgroup_get()

Since it's illegal for the block address of a non-inlined pcluster to
be zero, add check here to avoid registering the pcluster which would
be leaked.

Fixes: cecf864d3d76 ("erofs: support inline data decompression")
Reported-by: syzbot+6f8cd9a0155b366d227f@syzkaller.appspotmail.com
Signed-off-by: Chen Zhongjin <chenzhongjin@huawei.com>
Reviewed-by: Yue Hu <huyue2@coolpad.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Link: https://lore.kernel.org/r/Y42Kz6sVkf+XqJRB@debian
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>