operands are consumed so use the appropriate constraint modifier.
Before this change GCC used one register for both an input and an
unrelated output operand of in_addword(), causing the input to be
overwritten before it was consumed and thus breaking in_addword().
For in_cksum_hdr() and in_pseudo() this change is more or less
cosmetic.
- Fix a misspelling in a nearby comment.
Reported & tested by: yongari
MFC after: 1 week