datapath: Fix build breakage on kernel 2.6.40

datapath: Fix build breakage on kernel 2.6.40

Today i played with openvswitch on my workstation with kernel 2.6.40 and found that it break when i built. The
+issue is introduced by commit ceb176fdb72bb7ce90debc66e1eeb1d25823d30a

Below is the error log:

from /home/zwu/work/virt/openvswitch/datapath/linux/genetlink-brcompat.c:10:
/home/zwu/work/virt/openvswitch/datapath/linux/compat/include/linux/skbuff.h:243:20: error: redefinition of
+‘skb_reset_mac_len’
include/linux/skbuff.h:1259:20: note: previous definition of ‘skb_reset_mac_len’ was here
make[5]: *** [/home/zwu/work/virt/openvswitch/datapath/linux/genetlink-brcompat.o] Error 1
make[4]: *** [_module_/home/zwu/work/virt/openvswitch/datapath/linux] Error 2
make[4]: Leaving directory `/usr/src/kernels/2.6.40.6-0.fc15.x86_64'
make[3]: *** [default] Error 2
make[3]: Leaving directory `/home/zwu/work/virt/openvswitch/datapath/linux'
make[2]: *** [all-recursive] Error 1
make[2]: Leaving directory `/home/zwu/work/virt/openvswitch/datapath'
make[1]: *** [all-recursive] Error 1
make[1]: Leaving directory `/home/zwu/work/virt/openvswitch'
make: *** [all] Error 2

Signed-off-by: Zhi Yong Wu <zwu.kernel@gmail.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>