erofs: fix offset truncation when shifting pgoff on 32-bit platforms

erofs: fix offset truncation when shifting pgoff on 32-bit platforms

On 32-bit platforms, pgoff_t is 32 bits wide, so left-shifting
large arbitrary pgoff_t values by PAGE_SHIFT performs 32-bit arithmetic
and silently truncates the result for pages beyond the 4 GiB boundary.

Cast the page index to loff_t before shifting to produce a correct
64-bit byte offset.

Fixes: 386292919c25 ("erofs: introduce readmore decompression strategy")
Fixes: 307210c262a2 ("erofs: verify metadata accesses for file-backed mounts")
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>