zdtm/inotify_system_nodel.c: need to print mask of events for UNKNOWN event

Need to print a mask of events for UNKNOWN event in inotify_system_nodel.c
and inotify_system.c tests. The current error message in this case is too vague:
inotify_system_nodel.c:266: got unexpected event UNKNOWN

Signed-off-by: vkonyashkin <vkonyashkin@parallels.com>
Acked-by: Andrew Vagin <avagin@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>