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

tools/testing/selftests/bpf/progs/get_func_ip_test.c

@@ -5,13 +5,12 @@
 
 char _license[] SEC("license") = "GPL";
 
-extern const void bpf_fentry_test1 __ksym;
+extern int bpf_fentry_test1(int a) __ksym;
+extern int bpf_modify_return_test(int a, int *b) __ksym;
+
 extern const void bpf_fentry_test2 __ksym;
 extern const void bpf_fentry_test3 __ksym;
 extern const void bpf_fentry_test4 __ksym;
-extern const void bpf_modify_return_test __ksym;
-extern const void bpf_fentry_test6 __ksym;
-extern const void bpf_fentry_test7 __ksym;
 
 extern bool CONFIG_X86_KERNEL_IBT __kconfig __weak;