V4L/DVB (6276): V4L: videobuf-core.c lock before streaming check

The reading/streaming fields are used for mutual exclusion of the queue and
should be protected by the queue lock.

Signed-off-by: Brandon Philips <bphilips@suse.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>
http://thread.gmane.org/gmane.comp.video.video4linux/34978/focus=34981


Reviewed-by: Ricardo Cerqueira <v4l@cerqueira.org>