datapath: Fix warning building datapath on pre-2.6.24 kernels.

The Linux 'min' macro checks that its arguments have the same type, and
if not the compiler reports a message about incompatible pointer types.
On pre-2.6.24 kernels skb_headroom() returns int, so this code was
firing a warning:
       unsigned headroom = max(min_headroom, skb_headroom(skb));

This commit makes skb_headroom() return an unsigned int regardless of
kernel version.