Use "asm volatile" rather than "asm" for instruction sequences
that modify condition codes (the carry bit, in this case). Without
"volatile", the compiler might add the inline assembler instructions
between unrelated code which also uses condition codes, modifying the
latter.
This prevents the TCP pseudo header checksum calculation done in
tcp_output() from having effects on other conditions when compiled
with GCC 4.2.1 at "-O2" and "options INET6" left out. [1]

Reported & tested by: Boris Kochergin [1]
MFC after: 3 days