net: check for TUNSETOFFLOAD support before trying to enable offload features This avoids the "TUNSETOFFLOAD ioctl() failed: Invalid argument" message on kernels without TUNSETOFFLOAD support. Signed-off-by: Pierre Riteau <Pierre.Riteau@irisa.fr> Signed-off-by: Mark McLoughlin <markmc@redhat.com> Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>