selftests: net: name bpf objects consistently and simplify Makefile

The BPF sources moved with bpf_offload.py have a suffix of .bpf.c
which seems to be useful convention. Rename the 2 other BPF sources
we had. Use wildcard in the Makefile, since we can match all those
files easily now.

Link: https://lore.kernel.org/r/20240423183542.3807234-2-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>