virtio-net: Fix size check in dhclient workaround

virtio-net: Fix size check in dhclient workaround

work_around_broken_dhclient() accesses IP and UDP headers to detect
relevant packets and to calculate checksums, but it didn't check if
the packet has size sufficient to accommodate them, causing out-of-bound
access hazards. Fix this by correcting the size requirement.

Fixes: 1d41b0c1ec66 ("Work around dhclient brokenness")
Cc: qemu-stable@nongnu.org
Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Signed-off-by: Jason Wang <jasowang@redhat.com>
(cherry picked from commit a8575f7fb2f213e6690b23160b04271d47fdfaa8)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>