erofs: convert all uncompressed cases to iomap

Since tail-packing inline has been supported by iomap now, let's
convert all EROFS uncompressed data I/O to iomap, which is pretty
straight-forward.

Cc: linux-fsdevel@vger.kernel.org
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>