nodedev: Only set up mdevctl monitors if mdevctl.d exist

nodedev: Only set up mdevctl monitors if mdevctl.d exist

During its initialization, the nodedev driver tries to set up
monitors for /etc/mdevctl.d directory, so that it can register
mdevs as they come and go. However, if the file doesn't exist
there is nothing to monitor and therefore we can exit early. In
fact, we have to otherwise monitorFileRecursively() fails and
whole driver initialization fails with it.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Pavel Hrdina <phrdina@redhat.com>