selftests/bpf: fix selftest after random: Urandom_read tracepoint removal

14c17463

 ("random: remove unused tracepoints") removed all the
tracepoints from drivers/char/random.c, one of which,
random:urandom_read, was used by stacktrace_build_id selftest to trigger
stack trace capture.

Fix breakage by switching to kprobing urandom_read() function.

Suggested-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20220325225643.2606-1-andrii@kernel.org