[NET]: Make second arg to skb_reserved() signed.

Some subsystems, such as PPP, can send negative values
here.  It just happened to work correctly on 32-bit with
an unsigned value, but on 64-bit this explodes.

Figured out by Paul Mackerras based upon several PPP crash
reports.

Signed-off-by: David S. Miller <davem@davemloft.net>