samples/bpf, selftests/bpf: Use bpf_probe_read_kernel

A handful of samples and selftests fail to build on s390, because
after commit 0ebeea8ca8 ("bpf: Restrict bpf_probe_read{, str}()
only to archs where they work") bpf_probe_read is not available
anymore.

Fix by using bpf_probe_read_kernel.

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200720114806.88823-1-iii@linux.ibm.com

samples/bpf/offwaketime_kern.c

@@ -12,7 +12,12 @@
 #include <bpf/bpf_helpers.h>
 #include <bpf/bpf_tracing.h>
 
-#define _(P) ({typeof(P) val; bpf_probe_read(&val, sizeof(val), &P); val;})
+#define _(P)                                                                   \
+	({                                                                     \
+		typeof(P) val;                                                 \
+		bpf_probe_read_kernel(&val, sizeof(val), &(P));                \
+		val;                                                           \
+	})
 
 #define MINBLOCK_US	1