media: staging/intel-ipu3: Implement lock for stream on/off operations

Currently concurrent stream off operations on ImgU nodes are not
synchronized, leading to use-after-free bugs (as reported by KASAN).

[  250.090724] BUG: KASAN: use-after-free in
ipu3_dmamap_free+0xc5/0x116 [ipu3_imgu]
[  250.090726] Read of size 8 at addr ffff888127b29bc0 by task
yavta/18836
[  250.090731] Hardware name: HP Soraka/Soraka, BIOS
Google_Soraka.10431.17.0 03/22/2018
[  250.090732] Call Trace:
[  250.090735]  dump_stack+0x6a/0xb1
[  250.090739]  print_address_description+0x8e/0x279
[  250.090743]  ? ipu3_dmamap_free+0xc5/0x116 [ipu3_imgu]
[  250.090746]  kasan_report+0x260/0x28a
[  250.090750]  ipu3_dmamap_free+0xc5/0x116 [ipu3_imgu]
[  250.090754]  ipu3_css_pool_cleanup+0x24/0x37 [ipu3_imgu]
[  250.090759]  ipu3_css_pipeline_cleanup+0x61/0xb9 [ipu3_imgu]
[  250.090763]  ipu3_css_stop_streaming+0x1f2/0x321 [ipu3_imgu]
[  250.090768]  imgu_s_stream+0x94/0x443 [ipu3_imgu]
[  250.090772]  ? ipu3_vb2_buf_queue+0x280/0x280 [ipu3_imgu]
[  250.090775]  ? vb2_dma_sg_unmap_dmabuf+0x16/0x6f [videobuf2_dma_sg]
[  250.090778]  ? vb2_buffer_in_use+0x36/0x58 [videobuf2_common]
[  250.090782]  ipu3_vb2_stop_streaming+0xf9/0x135 [ipu3_imgu]

Implemented a lock to synchronize imgu stream on / off operations and
the modification of streaming flag (in struct imgu_device), to prevent
these issues.

Reported-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Suggested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Rajmohan Mani <rajmohan.mani@intel.com>
Signed-off-by: Bingbu Cao <bingbu.cao@intel.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>