net: ip: net_pkt: only reserve L2 header for TX case

Only reserve L2 header for TX case when allocating net buffer,
as for RX case, all the received headers are already in linear
buffer when the driver receives the data, and reserve extra L2
header for RX case may exceed the default buffer size.

Signed-off-by: Maochen Wang <maochen.wang@nxp.com>