kernel: l2tp: Fix oops in pppol2tp_xmit

[Eugene Teo <eugeneteo () kernel sg>]

"When transmitting L2TP frames, we derive the outgoing interface's UDP 
checksum hardware assist capabilities from the tunnel dst dev. This can 
sometimes be NULL, especially when routing protocols are used and 
routing changes occur. This patch just checks for NULL dst or dev 
pointers when checking for netdev hardware assist features.

    BUG: unable to handle kernel NULL pointer dereference at 0000000c
    IP: [<f89d074c>] pppol2tp_xmit+0x341/0x4da [pppol2tp]
    *pde = 00000000
    Oops: 0000 [#1] SMP
    last sysfs file: /sys/class/net/lo/operstate
[...]"

Introduced in ffcebb16 (v2.6.29-rc1~581), fixed in 3feec909 (fixed in 
v2.6.34-rc2). (It was later split into different files in commit 
fd558d18 v2.6.35-rc1).

I'm not requesting a CVE name for this because it did not affect any of 
our supported kernels. FYI.

Thanks, Eugene
--
main(i) { putchar(182623909 >> (i-1) * 5&31|!!(i<7)<<6) && main(++i); }