tun: avoid BUG, dump packet on GSO errors

There are still some LRO cards that cause GSO errors in tun,
and BUG on this is an unfriendly way to tell the admin
to disable LRO.

Further, experience shows we might have more GSO bugs lurking.
See https://bugzilla.kernel.org/show_bug.cgi?id=16413
as a recent example.
dumping a packet will make it easier to figure it out.

Replace BUG with warning+dump+drop the packet to make
GSO errors in tun less critical and easier to debug.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Tested-by: Alex Unigovsky <unik@compot.ru>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>