CVE-2026-68260

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

drm/imagination: acquire vm_ctx->lock before mapping memory to GPU VM

The drm gpuvm code doesn't protect find operation against map operation,
and the driver needs to ensure a map operation shouldn't happen when a
find operation is in progress.

In some cases a find operation will be in progress when doing map/unmap
operations, and the find operation will do a NULL pointer dereference.

An example of the stack trace of such NULL dereference is shown below:

```
Unable to handle kernel access to user memory without uaccess routines at
virtual address 0000000000000010

[<ffffffff01e989d4>] drm_gpuva_find+0x28/0x6c [drm_gpuvm]
[<ffffffff01ed3a40>] pvr_vm_unmap+0x34/0x68 [powervr]
[<ffffffff01ec69da>] pvr_ioctl_vm_unmap+0x2e/0x50 [powervr]
[<ffffffff8080ce0a>] drm_ioctl_kernel+0x8e/0xdc
[<ffffffff8080d016>] drm_ioctl+0x1be/0x3e0
[<ffffffff802bec3e>] __riscv_sys_ioctl+0xba/0xc4
[<ffffffff80d858b2>] do_trap_ecall_u+0x23e/0x3f4
[<ffffffff80d92288>] handle_exception+0x168/0x174
```

As all occurences of drm_gpuva_find*() are already guarded by
vm_ctx->lock, make pvr_vm_map() to acquire this lock to prevent
disturbing any find operation. This fixes the NULL deference problem in
drm_gpuva_find*().