V4L/DVB: dvb_demux: Don't use vmalloc at dvb_dmx_swfilter_packet

As dvb_dmx_swfilter_packet() is protected by a spinlock, it shouldn't sleep.
However, vmalloc() may call sleep. So, move the initialization of
dvb_demux::cnt_storage field to a better place.

Reviewed-by: Andy Walls <awalls@radix.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>