[media] vb2-core: add support for count == 0 in create_bufs

This also fixes incorrect error handling in create_bufs: the return code
of __vb2_queue_alloc is the number of allocated buffers, and not a
traditional error code.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>