net: pkt: Alloc headroom also for variable size data buffers

The headroom was not taken into account for variable size data
buffers when CONFIG_NET_L2_ETHERNET_RESERVE_HEADER was enabled.

Add a test case for it to make sure the reserve allocation works
properly.

Fixes #84053

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>