selftests/bpf: test_xdp_vlan: Rename BPF sections

The __load() helper expects BPF sections to be names 'xdp' or 'tc'

Rename BPF sections so they can be loaded with the __load() helper in
upcoming patch.
Rename the BPF functions with their previous section's name.
Update the 'ip link' commands in the script to use the program name
instead of the section name to load the BPF program.

Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Signed-off-by: Bastien Curutchet (eBPF Foundation) <bastien.curutchet@bootlin.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20250221-xdp_vlan-v1-1-7d29847169af@bootlin.com