datapath: Check for proto_data_valid member instead of kernel version.

Commit 5ef800a69 "datapath: Copy Xen's checksumming fields when doing
skb_copy" should copy proto_data_valid between sk_buffs when that field
is present.  However the check for CONFIG_XEN plus kernel version 2.6.18
isn't sufficient, because SLES 11 kernels are version 2.6.27 but do have
this field.

This commit adds a configure-time check for the presence of the member
instead of attempting to guess based on the kernel version.

Thanks to Ian Campbell for reporting this problem.

CC: <Ian.Campbell@citrix.com>