perf probe: Clarify error message about not finding kernel modules debuginfo

'perf probe' supports using just the kernel module name, but that will
work only when the module is loaded, or using the full pathname to the
file with the DWARF debug info, but the warning was cryptic:

Before:

  # perf probe -m cls_flower -L fl_change
  Failed to find the path for cls_flower: No such file or directory
    Error: Failed to show lines.
  #

After:

  # perf probe -m cls_flower -L fl_change
  Module cls_flower is not loaded, please specify its full path name.
    Error: Failed to show lines.
  # perf probe -m /lib/modules/5.0.0-rc7+/kernel/net/sched/cls_flower.ko -L fl_change | head -7
  <fl_change@/home/acme/git/linux/net/sched/cls_flower.c:0>
        0  static int fl_change(struct net *net, struct sk_buff *in_skb,
         		       struct tcf_proto *tp, unsigned long base,
         		       u32 handle, struct nlattr **tca,
         		       void **arg, bool ovr, struct netlink_ext_ack *extack)
        4  {
        5  	struct cls_fl_head *head = rtnl_dereference(tp->root);
  #

The behaviour doesn't change when the module is loaded:

  # modprobe cls_flower
  # perf probe -m cls_flower -L fl_change | head -7
  <fl_change@/home/acme/git/linux/net/sched/cls_flower.c:0>
        0  static int fl_change(struct net *net, struct sk_buff *in_skb,
                               struct tcf_proto *tp, unsigned long base,
                               u32 handle, struct nlattr **tca,
                               void **arg, bool ovr, struct netlink_ext_ack *extack)
        4  {
        5         struct cls_fl_head *head = rtnl_dereference(tp->root);
  #

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Marcelo Ricardo Leitner <mleitner@redhat.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-q4njvk9mshra00jacqjbzfn5@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/probe-event.c

@@ -472,9 +472,12 @@ static struct debuginfo *open_debuginfo(const char *module, struct nsinfo *nsi,
 					strcpy(reason, "(unknown)");
 			} else
 				dso__strerror_load(dso, reason, STRERR_BUFSIZE);
-			if (!silent)
-				pr_err("Failed to find the path for %s: %s\n",
-					module ?: "kernel", reason);
+			if (!silent) {
+				if (module)
+					pr_err("Module %s is not loaded, please specify its full path name.\n", module);
+				else
+					pr_err("Failed to find the path for the kernel: %s\n", reason);
+			}
 			return NULL;
 		}
 		path = dso->long_name;