dccp: Fix out of bounds access in DCCP error handler

In the Linux kernel, the following vulnerability has been resolved:

dccp: Fix out of bounds access in DCCP error handler

There was a previous attempt to fix an out-of-bounds access in the DCCP

error handlers, but that fix assumed that the error handlers only want

to access the first 8 bytes of the DCCP header. Actually, they also look

at the DCCP sequence number, which is stored beyond 8 bytes, so an

explicit pskb_may_pull() is required.