sd-device: generate e better error code when trying to allocate sd_device for non-dir

Currently, for sysfs paths outside of /sys/devices/ we do better
checking if something is a suitable path: we check if it's actually a
directory, and if not return ENODEV.

Let's make the codepath for nodes *inside* of /sys/device/ similar:
let's also return ENODEV if the path supplied is not a directory.

Previously, we'd return ENOTDIR in that case, which is quite confusing I
think.

src/libsystemd/sd-device/sd-device.c

@@ -196,6 +196,10 @@ int device_set_syspath(sd_device *device, const char *_syspath, bool verify) {
                                          * Hence, use log_trace_errno() here. */
                                         return log_trace_errno(SYNTHETIC_ERRNO(ENODEV),
                                                                "sd-device: the uevent file \"%s/uevent\" does not exist.", syspath);
+                                if (errno == ENOTDIR)
+                                        /* Not actually a directory. */
+                                        return log_debug_errno(SYNTHETIC_ERRNO(ENODEV),
+                                                               "sd-device: the syspath \"%s\" is not a directory.", syspath);
 
                                 return log_debug_errno(errno, "sd-device: cannot find uevent file for %s: %m", syspath);
                         }