llc: Fix length check in llc_fixup_skb().

Fixes bugzilla #32872

The LLC stack pretends to support non-linear skbs but there is a
direct use of skb_tail_pointer() in llc_fixup_skb().

Use pskb_may_pull() to see if data_size bytes remain and can be
accessed linearly in the packet, instead of direct pointer checks.

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