optimization level (-march=pentium-mmx for example) does not insert
intermediate ops which would trash the carry.
Change both sys/i386/i386/in_cksum.c[1] and sys/i386/include/in_cksum.h.
To my best understanding the same problem was addressed in rev. 1.16
of src/sys/i386/include/in_cksum.h for just a single function 3y ago.
Reviewed by: jhb
Submitted by: Zhouyi ZHOU <zhouzhouyi FreeBSD.org> (intial version of [1])
MFC after: 5 days
PR: 115678, 69257