DST_QUEUE_OFF_BASE is applied to offset/mem_offset on MMAP capture buffers
only for the VIDIOC_QUERYBUF ioctl, while the userspace fields (including
offset/mem_offset) are filled in for VIDIOC_{QUERY,PREPARE,Q,DQ}BUF
ioctls. This leads to differences in the values presented to userspace.
If userspace attempts to mmap the capture buffer directly using values
from DQBUF, it will fail.

Move the code that applies the magic offset into a helper, and call
that helper from all four ioctl entry points.

[hverkuil: drop unnecessary '= 0' in v4l2_m2m_querybuf() for ret]

Fixes: 7f98639d ("V4L/DVB: add memory-to-memory device helper framework for videobuf")
Fixes: 908a0d7c ("[media] v4l: mem2mem: port to videobuf2")
Signed-off-by: Chen-Yu Tsai <wenst@chromium.org>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>