selftests/bpf: fix stacktrace_build_id with missing kprobe/urandom_read

Kernel function urandom_read is replaced with urandom_read_iter.
Therefore, kprobe on urandom_read is not working any more:

[root@eth50-1 bpf]# ./test_progs -n 161
test_stacktrace_build_id:PASS:skel_open_and_load 0 nsec
libbpf: kprobe perf_event_open() failed: No such file or directory
libbpf: prog 'oncpu': failed to create kprobe 'urandom_read+0x0' \
perf event: No such file or directory
libbpf: prog 'oncpu': failed to auto-attach: -2
test_stacktrace_build_id:FAIL:attach_tp err -2
161 stacktrace_build_id:FAIL

Fix this by replacing urandom_read with urandom_read_iter in the test.

Fixes: 1b388e7765f2 ("random: convert to using fops->read_iter()")
Reported-by: Mykola Lysenko <[e-mail]>
Signed-off-by: Song Liu <[e-mail]>
Acked-by: David Vernet <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

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

···
 	__type(value, stack_trace_t);
 } stack_amap SEC(".maps");
 
-SEC("kprobe/urandom_read")
+SEC("kprobe/urandom_read_iter")
 int oncpu(struct pt_regs *args)
 {
 	__u32 max_len = sizeof(struct bpf_stack_build_id)