ASoC: dapm: Fix UAF for snd_soc_pcm_runtime object

When using kernel with the following extra config,

- CONFIG_KASAN=y
- CONFIG_KASAN_GENERIC=y
- CONFIG_KASAN_INLINE=y
- CONFIG_KASAN_VMALLOC=y
- CONFIG_FRAME_WARN=4096

kernel detects that snd_pcm_suspend_all() access a freed
'snd_soc_pcm_runtime' object when the system is suspended, which
leads to a use-after-free bug:

[ 52.047746] BUG: KASAN: use-after-free in snd_pcm_suspend_all+0x1a8/0x270
[ 52.047765] Read of size 1 at addr ffff0000b9434d50 by task systemd-sleep/2330

[ 52.047785] Call trace:
[ 52.047787] dump_backtrace+0x0/0x3c0
[ 52.047794] show_stack+0x34/0x50
[ 52.047797] dump_stack_lvl+0x68/0x8c
[ 52.047802] print_address_description.constprop.0+0x74/0x2c0
[ 52.047809] kasan_report+0x210/0x230
[ 52.047815] __asan_report_load1_noabort+0x3c/0x50
[ 52.047820] snd_pcm_suspend_all+0x1a8/0x270
[ 52.047824] snd_soc_suspend+0x19c/0x4e0

The snd_pcm_sync_stop() has a NULL check on 'substream->runtime' before
making any access. So we need to always set 'substream->runtime' to NULL
everytime we kfree() it.

Fixes: a72706ed8208 ("ASoC: codec2codec: remove ephemeral variables")
Signed-off-by: robelin <robelin@nvidia.com>
Signed-off-by: Sameer Pujar <spujar@nvidia.com>
Link: https://patch.msgid.link/20240823144342.4123814-2-spujar@nvidia.com
Signed-off-by: Mark Brown <broonie@kernel.org>