tools lib fs debugfs: Check if debugfs is mounted when handling ENOENT

If debugfs was already mounted, then its a matter of not finding the
tracepoint, tell the user that perhaps a CONFIG_ setting is not enabled.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Don Zickus <dzickus@redhat.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-6chfytoflyx3jwfqm7ebltu0@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/lib/api/fs/debugfs.c

@@ -106,6 +106,13 @@ int debugfs__strerror_open(int err, char *buf, size_t size, const char *filename
 
 	switch (err) {
 	case ENOENT:
+		if (debugfs_found) {
+			snprintf(buf, size,
+				 "Error:\tFile %s/%s not found.\n"
+				 "Hint:\tPerhaps this kernel misses some CONFIG_ setting to enable this feature?.\n",
+				 debugfs_mountpoint, filename);
+			break;
+		}
 		snprintf(buf, size, "%s",
 			 "Error:\tUnable to find debugfs\n"
 			 "Hint:\tWas your kernel compiled with debugfs support?\n"