samples: bpf: fix: broken sample regarding removed function

Currently, running sample "task_fd_query" and "tracex3" occurs the
following error. On kernel v5.0-rc* this sample will be unavailable
due to the removal of function 'blk_start_request' at commit "a1ce35fa".
(function removed, as "Single Queue IO scheduler" no longer exists)

$ sudo ./task_fd_query
failed to create kprobe 'blk_start_request' error 'No such file or
directory'

This commit will change the function 'blk_start_request' to
'blk_mq_start_request' to fix the broken sample.
Signed-off-by: Daniel T. Lee <danieltimlee@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

samples/bpf/task_fd_query_kern.c

@@ -4,7 +4,7 @@
 #include <uapi/linux/bpf.h>
 #include "bpf_helpers.h"
 
-SEC("kprobe/blk_start_request")
+SEC("kprobe/blk_mq_start_request")
 int bpf_prog1(struct pt_regs *ctx)
 {
 	return 0;

samples/bpf/task_fd_query_user.c

@@ -311,7 +311,7 @@ int main(int argc, char **argv)
 	}
 
 	/* test two functions in the corresponding *_kern.c file */
-	CHECK_AND_RET(test_debug_fs_kprobe(0, "blk_start_request",
+	CHECK_AND_RET(test_debug_fs_kprobe(0, "blk_mq_start_request",
 					   BPF_FD_TYPE_KPROBE));
 	CHECK_AND_RET(test_debug_fs_kprobe(1, "blk_account_io_completion",
 					   BPF_FD_TYPE_KRETPROBE));

samples/bpf/tracex3_kern.c

@@ -20,7 +20,7 @@ struct bpf_map_def SEC("maps") my_map = {
 /* kprobe is NOT a stable ABI. If kernel internals change this bpf+kprobe
  * example will no longer be meaningful
  */
-SEC("kprobe/blk_start_request")
+SEC("kprobe/blk_mq_start_request")
 int bpf_prog1(struct pt_regs *ctx)
 {
 	long rq = PT_REGS_PARM1(ctx);