net: Change x86_64 add32_with_carry to allow memory operand

Note add32_with_carry(a, b) is suboptimal, as it forces
a and b in registers.

b could be a memory or a register operand.

Signed-off-by: Tom Herbert <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

arch/x86/include/asm/checksum_64.h

···
 	asm("addl %2,%0\n\t"
 	    "adcl $0,%0"
 	    : "=r" (a)
-	    : "0" (a), "r" (b));
+	    : "0" (a), "rm" (b));
 	return a;
 }