dpif-netlink: Drop oversized packets instead of assert-failing.

dpif-netlink: Drop oversized packets instead of assert-failing.

A packet sent to a Netlink datapath has to fit within a Netlink attribute.
Until now, this was only checked in an assertion inside the Netlink code,
which meant that trying to send a too-large packet (approximate 64 kB or
larger) would assert-fail.  It's better to just drop those packets, which
this commit does.

Reported-by: Shuping Cui <scui@redhat.com>
Reported-by: Jiri Benc <jbenc@redhat.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>