xen/netback: fix build warning

Commit ad7f402a ("xen/netback: Ensure protocol headers don't fall in
the non-linear area") introduced a (valid) build warning. There have
even been reports of this problem breaking networking of Xen guests.

Fixes: ad7f402a ("xen/netback: Ensure protocol headers don't fall in the non-linear area")
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Tested-by: Jason Andryuk <jandryuk@gmail.com>
Signed-off-by: Juergen Gross <jgross@suse.com>