net sched actions: use sizeof operator for buffer length

Replace constant integer with sizeof() to clearly indicate
the destination buffer length in skb_header_pointer() calls.

Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: Roman Mashak <mrv@mojatatu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>