V4L/DVB (5123): Buf_qbuf: fix: videobuf_queue->stream corruption and lockup

We are doing ->buf_prepare(buf) before adding buf to q->stream list. This
means that videobuf_qbuf() should not try to re-add a STATE_PREPARED buffer.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>