media: v4l2-mem2mem: Correct return type for mem2mem buffer helpers

media: v4l2-mem2mem: Correct return type for mem2mem buffer helpers

This commit changes the return 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

which currently return void pointer.

In every case, the actual return type is a struct vb2_v4l2_buffer
pointer. Change the return type of the listed functions,
so type checking can be properly used.

Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
[hverkuil-cisco@xs4all.nl: clean up line-too-long checkpatch warnings]
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>