general protection fault in pppol2tp_connect

Hello,

syzbot has tested the proposed patch and the reproducer did not trigger  
crash:

Reported-and-tested-by:  
syzbot+6e6a5ec8de31a94cd015@syzkaller.appspotmail.com

Note: the tag will also help syzbot to understand when the bug is fixed.

Tested on  
https://git.kernel.org/pub/scm/linux/kernel/git/davem/net.git/master commit
79c0ef3e85c015b0921a8fd5dd539d1480e9cd6c (Mon Feb 19 19:58:19 2018 +0000)
Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net

compiler: gcc (GCC) 7.1.1 20170620
Patch is attached.
Kernel config is attached.


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