erofs: fix fsdax partition offset handling

After seeking time on testing today upstream fsdax, I found it
actually doesn't work well as below:

[  186.492983] ------------[ cut here ]------------
[  186.493629] WARNING: CPU: 1 PID: 205 at fs/iomap/iter.c:33 iomap_iter+0x2f6/0x310

The problem is that m_dax_part_off should be applied to physical
addresses and very sorry about that I didn't catch this eariler.

Anyway, let's fix it up now. Also, I need to find a way to set up
a standalone testcase to look after this later.

Link: https://lore.kernel.org/r/20220113051845.244461-1-hsiangkao@linux.alibaba.com
Fixes: de205114

 ("fsdax: shift partition offset handling into the file systems")
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>