[LLC]: Use pskb_trim_rcsum() in llc_fixup_skb().

Kernel Bugzilla #6409

If we use plain skb_trim(), that's wrong, because if
the SKB is cloned, and it can be because we unshared
it in the caller, we have to allow reallocation.  The
pskb_trim*() family of routines is therefore the most
appropriate here.

Signed-off-by: David S. Miller <davem@davemloft.net>