selftests/bpf: Skip livepatch test when prerequisites are missing

selftests/bpf: Skip livepatch test when prerequisites are missing

livepatch_trampoline relies on livepatch sysfs and livepatch-sample.ko.
When CONFIG_LIVEPATCH is disabled or the samples module isn't built, the
test fails with ENOENT and causes false failures in minimal CI configs.

Skip the test when livepatch sysfs or the sample module is unavailable.
Also avoid writing to livepatch sysfs when it's not present.

Signed-off-by: Sun Jian <sun.jian.kdev@gmail.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/r/20260309104448.817401-1-sun.jian.kdev@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>