[lucid] cherry-pick BUG->WARN change in tun

[dann frazier]

Please consider pulling the following change into lucid. This resolves
LP #698883.

SRU Justification:
 Avoids crash
Impact:
 BUG() causes system to crash
Fix:
 Upstream reduced BUG() to a WARN_ON_ONCE w/ additional diagnostic info.
 Upstream commit ef3db4a5954281bc1ea49a4739c88eaea091dc71.
Testcase:
 A tractable case would presumably require packet injection. Otherwise, a long
 stable run on a configuration that has hit this problem before.

This was originally reported by a customer in restricted LP #680356. A
custom kernel w/ this fix was applied and neither the issue nor a
regression has been observed in the following month.

Upstream change follows - it cherry-picks cleanly into the lucid git tree.

commit ef3db4a5954281bc1ea49a4739c88eaea091dc71
Author: Michael S. Tsirkin <mst at redhat.com>
Date: Wed Jul 21 04:32:45 2010 +0000

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