Properly place #ifdef INET and #ifdef INET6 around variable declarations
and code to make the code compile.

Give the function seems to be slightly mixed with csum and tso,
make it non-fatal if we try to setup thing on a kernel without IP
support.  In practise the printf on the console will probably still
make your machine unhappy.

MFC after:	3 days
X-MFC with:	r270755
These functions are #defined to "osdep" specific names without the "_mem"
extension.  Provide prototypes for the actual implementations.
Correct function arguments to match the implementations.

MFC after:	3 days
X-MFC with:	r270755