net: Using single TX net buf for variable size buffer causes issues

[krish2718]

Describe the bug
In nRF7002DK, when using two features CONFIG_NET_BUF_VARIABLE_DATA_SIZE and CONFIG_NET_L2_ETHERNET_RESERVE_HEADER, data packets are being dropped with below info causing Wi-Fi connection issues as those data packets are EAPoL.

[00:03:51.467,285] <dbg> net_ethernet: ethernet_fill_header: (tx_q[0]): Making room for link header 14 bytes
[00:03:51.479,705] <dbg> net_ethernet: ethernet_fill_header: (tx_q[0]): iface 1 (0x20001238) src F4:CE:36:00:6C:A9 dst FF:FF:FF:FF:FF:FF type 0x86dd len 14
[00:03:51.497,863] <err> net_if: Cannot join mcast address 224.0.0.1 for 1 (-105)
[00:03:51.508,544] <dbg> net_ethernet: ethernet_fill_header: (tx_q[0]): Making room for link header 14 bytes
[00:03:51.520,965] <dbg> net_ethernet: ethernet_fill_header: (tx_q[0]): iface 1 (0x20001238) src F4:CE:36:00:6C:A9 dst FF:FF:FF:FF:FF:FF type 0x86dd len 14
[00:03:51.542,358] <dbg> net_ethernet: ethernet_recv: (rx_q[0]): iface 1 (0x20001238) src C8:7F:54:38:0C:08 dst F4:CE:36:00:6C:A9 type 0x888e len 113
[00:03:51.566,284] <err> net_ctx: Available payload buffer (107) is not enough for requested DGRAM (121)
[00:03:51.578,674] <err> wpa_supp: l2_packet_send - sendto: Not enough space
Connected
Connection request failed (2)

Using fixed size buffers connection goes through.

To Reproduce

Steps to reproduce the behavior:

1. Build and flash samples/net/wifi/shell for nrf7002DK and enable above two features.
2. Try to connect a secure Wi-FI AP
3. Connection fails

Expected behavior
Wi-Fi connection should go through using those two features.

Impact
Low (new and non-default feature)

Logs and console output
shared above.

Environment (please complete the following information):

• OS:Linux
• Toolchain: zephyr-sdk-0.17.0)
• Commit SHA or Version used: main

Additional context
None