[media] exynos-gsc: cleanup m2m src and dst vb2 queues on STREAMOFF

Media drivers that use the videobuf2 framework have to give back to vb2
all the buffers that received from vb2 using its .buf_queue callback.

But the exynos-gsc driver isn't doing a proper cleanup so vb2 complains
that the number of buffers enqueued and received are not balanced:

WARNING: CPU: 2 PID: 660 at drivers/media/v4l2-core/videobuf2-core.c:1654 __vb2_queue_cancel+0xec/0x150 [videobuf2_core]
Modules linked in: mwifiex_sdio mwifiex uvcvideo exynos_gsc videobuf2_vmalloc s5p_mfc s5p_jpeg
CPU: 2 PID: 660 Comm: lt-gst-validate Not tainted 4.8.0
Hardware name: SAMSUNG EXYNOS (Flattened Device Tree)
[<c010e24c>] (unwind_backtrace) from [<c010af30>] (show_stack+0x10/0x14)
[<c010af30>] (show_stack) from [<c03291a4>] (dump_stack+0x88/0x9c)
[<c03291a4>] (dump_stack) from [<c011a858>] (__warn+0xe8/0x100)
[<c011a858>] (__warn) from [<c011a920>] (warn_slowpath_null+0x20/0x28)
[<c011a920>] (warn_slowpath_null) from [<bf0b6ed0>] (__vb2_queue_cancel+0xec/0x150 [videobuf2_core])
[<bf0b6ed0>] (__vb2_queue_cancel [videobuf2_core]) from [<bf0b7464>] (vb2_core_streamoff+0x34/0x9c [videobuf2_core])
[<bf0b7464>] (vb2_core_streamoff [videobuf2_core]) from [<bf11b9e8>] (v4l2_m2m_streamoff+0x2c/0xe4 [v4l2_mem2mem])
[<bf11b9e8>] (v4l2_m2m_streamoff [v4l2_mem2mem]) from [<bf01b84c>] (__video_do_ioctl+0x298/0x30c [videodev])
[<bf01b84c>] (__video_do_ioctl [videodev]) from [<bf01b234>] (video_usercopy+0x174/0x4e8 [videodev])
[<bf01b234>] (video_usercopy [videodev]) from [<bf0165c8>] (v4l2_ioctl+0xc4/0xd8 [videodev])
[<bf0165c8>] (v4l2_ioctl [videodev]) from [<c01f291c>] (do_vfs_ioctl+0x9c/0x8f4)
[<c01f291c>] (do_vfs_ioctl) from [<c01f31a8>] (SyS_ioctl+0x34/0x5c)
[<c01f31a8>] (SyS_ioctl) from [<c01078c0>] (ret_fast_syscall+0x0/0x3c)

Fix this by passing back to vb2 all the received buffers that were not
processed.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>