drm/panthor: Lock XArray when getting entries for the VM

Similar to commit cac0757 ("drm/panthor: Fix race when converting
group handle to group object") we need to use the XArray's internal
locking when retrieving a vm pointer from there.

v2: Removed part of the patch that was trying to protect fetching
the heap pointer from XArray, as that operation is protected by
the @pool->lock.

Fixes: 647810e ("drm/panthor: Add the MMU/VM logical block")
Reported-by: Jann Horn <jannh@google.com>
Cc: stable@vger.kernel.org
Signed-off-by: Liviu Dudau <liviu.dudau@arm.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Reviewed-by: Steven Price <steven.price@arm.com>
Signed-off-by: Steven Price <steven.price@arm.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20241106185806.389089-1-liviu.dudau@arm.com

Author: dliviu

drivers/gpu/drm/panthor/panthor_mmu.c

@@ -1580,7 +1580,9 @@ panthor_vm_pool_get_vm(struct panthor_vm_pool *pool, u32 handle)
 {
 	struct panthor_vm *vm;
 
+	xa_lock(&pool->xa);
 	vm = panthor_vm_get(xa_load(&pool->xa, handle));
+	xa_unlock(&pool->xa);
 
 	return vm;
 }