[NET]: Remove redundant code in net/core/filter.c

skb_header_pointer handles linear and non-linear data, no need to handle
linear data again.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>