erofs: fix block count report when 48-bit layout is on

Fix incorrect shift order when combining the 48-bit block count.

Fixes: 2e1473d5 ("erofs: implement 48-bit block addressing for unencoded inodes")
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20250807082019.3093539-1-hsiangkao@linux.alibaba.com