6lowpan: iphc: add support for stateful compression

Hello Alexander Aring,

The patch 5609c185f24d: "6lowpan: iphc: add support for stateful compression" from Feb 22, 2016, leads to the following static checker warning:

	net/6lowpan/iphc.c:815 lowpan_compress_ctx_addr()
	warn: signedness bug returning '(-22)'

net/6lowpan/iphc.c
812 }
813
814 WARN_ON_ONCE("context found but no address mode matched\n");

This isn't correct. WARN_ON_ONCE() takes a condition argument, not a string. It should be:

	WARN_ONCE(1, "context found but no address mode matched\n");

   815          return -EINVAL;
                ^^^^^^^^^^^^^^

This is a u8 function and none of the callers check for error.

   816  out:
   817  
   818          if (sam)
   819                  return lowpan_iphc_dam_to_sam_value[dam];
   820          else
   821                  return dam;
   822  }

regards,
dan carpenter