lib/scatterlist: use sg_phys() helper

This shorten the length of code in horizential direction, therefore is
easier to read.

Link: https://lkml.kernel.org/r/20241028182920.1025819-1-sui.jingfeng@linux.dev


Signed-off-by: Sui Jingfeng <sui.jingfeng@linux.dev>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>