- Honor return values from vb2_reqbufs on REQBUFS(0).
- Do not set the number of allocated buffers to 0 if userspace tries
to request buffers again without freeing them.
- There is no need to verify correct instance state on reqbufs, as we will
verify this in queue_setup().
- There is also no need to verify that vb2_reqbufs() was able to allocate enough
buffers (pb_count) and call buf_init on that many buffers (i.e. dst_buf_count
is at least pb_count), because this will be verified by second queue_setup()
call as well and vb2_reqbufs() will fail otherwise.
- Only verify state is MFCINST_INIT when allocating, not when freeing.