selftests: bpf: test_kmod.sh: check the module path before insmod

test_kmod.sh reported false failure when module not present.
Check test_bpf.ko is present in the path before loading it.

Two cases to be addressed here,
In the development process of test_bpf.c unit testing will be done by
developers by using "insmod $SRC_TREE/lib/test_bpf.ko"

On the other hand testers run full tests by installing modules on device
under test (DUT) and followed by modprobe to insert the modules accordingly.

Signed-off-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>