Fix the assigned type of mem2mem buffer handling API.
Namely, these functions:

 v4l2_m2m_next_buf
 v4l2_m2m_last_buf
 v4l2_m2m_buf_remove
 v4l2_m2m_next_src_buf
 v4l2_m2m_next_dst_buf
 v4l2_m2m_last_src_buf
 v4l2_m2m_last_dst_buf
 v4l2_m2m_src_buf_remove
 v4l2_m2m_dst_buf_remove

return a struct vb2_v4l2_buffer, and not a struct vb2_buffer.

Fixing this is necessary to fix the mem2mem buffer handling API,
changing the return to the correct struct vb2_v4l2_buffer instead
of a void pointer.
Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>