bpf: selftests: Fix fentry test kfunc prototypes

Some prototypes in progs/get_func_ip_test.c were not in line with how the
actual kfuncs are defined in net/bpf/test_run.c. This causes compilation
errors when kfunc prototypes are generated from BTF.

Fix by aligning with actual kfunc definitions.

Also remove two unused prototypes.

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Link: https://lore.kernel.org/r/1e68870e7626b7b9c6420e65076b307fc404a2f0.1718207789.git.dxu@dxuuu.xyz


Signed-off-by: Alexei Starovoitov <ast@kernel.org>