media: mediatek: vcodec: adding lock to protect decoder context list

In the Linux kernel, the following vulnerability has been resolved:

media: mediatek: vcodec: adding lock to protect decoder context list

Add a lock for the ctx_list, to avoid accessing a NULL pointer
within the 'vpu_dec_ipi_handler' function when the ctx_list has
been deleted due to an unexpected behavior on the SCP IP block.

Hardware name: Google juniper sku16 board (DT)
pstate: 20400005 (nzCv daif +PAN -UAO -TCO BTYPE=--)
pc : vpu_dec_ipi_handler+0x58/0x1f8 [mtk_vcodec_dec]
lr : scp_ipi_handler+0xd0/0x194 [mtk_scp]
sp : ffffffc0131dbbd0
x29: ffffffc0131dbbd0 x28: 0000000000000000
x27: ffffff9bb277f348 x26: ffffff9bb242ad00
x25: ffffffd2d440d3b8 x24: ffffffd2a13ff1d4
x23: ffffff9bb7fe85a0 x22: ffffffc0133fbdb0
x21: 0000000000000010 x20: ffffff9b050ea328
x19: ffffffc0131dbc08 x18: 0000000000001000
x17: 0000000000000000 x16: ffffffd2d461c6e0
x15: 0000000000000242 x14: 000000000000018f
x13: 000000000000004d x12: 0000000000000000
x11: 0000000000000001 x10: fffffffffffffff0
x9 : ffffff9bb6e793a8 x8 : 0000000000000000
x7 : 0000000000000000 x6 : 000000000000003f
x5 : 0000000000000040 x4 : fffffffffffffff0
x3 : 0000000000000020 x2 : ffffff9bb6e79080
x1 : 0000000000000010 x0 : ffffffc0131dbc08
Call trace:
vpu_dec_ipi_handler+0x58/0x1f8 [mtk_vcodec_dec (HASH:6c3f 2)]
scp_ipi_handler+0xd0/0x194 [mtk_scp (HASH:7046 3)]
mt8183_scp_irq_handler+0x44/0x88 [mtk_scp (HASH:7046 3)]
scp_irq_handler+0x48/0x90 [mtk_scp (HASH:7046 3)]
irq_thread_fn+0x38/0x94
irq_thread+0x100/0x1c0
kthread+0x140/0x1fc
ret_from_fork+0x10/0x30
Code: 54000088 f94ca50a eb14015f 54000060 (f9400108)
---[ end trace ace43ce36cbd5c93 ]---
Kernel panic - not syncing: Oops: Fatal exception
SMP: stopping secondary CPUs
Kernel Offset: 0x12c4000000 from 0xffffffc010000000
PHYS_OFFSET: 0xffffffe580000000
CPU features: 0x08240002,2188200c
Memory Limit: none