drm/xe/vm: Avoid reserving zero fences

The function xe_vm_prepare_vma was blindly accepting zero as the
number of fences and forwarded that to drm_exec_prepare_obj.

However, that leads to an out-of-bounds shift in the
dma_resv_reserve_fences() and while one could argue that the
dma_resv code should be robust against that, avoid attempting
to reserve zero fences.

Relevant stack trace:

[773.183188] ------------[ cut here ]------------
[773.183199] UBSAN: shift-out-of-bounds in ../include/linux/log2.h:57:13
[773.183241] shift exponent 64 is too large for 64-bit type 'long unsigned int'
[773.183254] CPU: 2 PID: 1816 Comm: xe_evict Tainted: G     U             6.8.0-rc3-xe #1
[773.183256] Hardware name: ASUS System Product Name/PRIME Z690-P D4, BIOS 2014 10/14/2022
[773.183257] Call Trace:
[773.183258]  <TASK>
[773.183260]  dump_stack_lvl+0xaf/0xd0
[773.183266]  dump_stack+0x10/0x20
[773.183283]  ubsan_epilogue+0x9/0x40
[773.183286]  __ubsan_handle_shift_out_of_bounds+0x10f/0x170
[773.183293]  dma_resv_reserve_fences.cold+0x2b/0x48
[773.183295]  ? ww_mutex_lock+0x3c/0x110
[773.183301]  drm_exec_prepare_obj+0x45/0x60 [drm_exec]
[773.183313]  xe_vm_prepare_vma+0x33/0x70 [xe]
[773.183375]  xe_vma_destroy_unlocked+0x55/0xa0 [xe]
[773.183427]  xe_vm_close_and_put+0x526/0x940 [xe]

Fixes: 2714d509 ("drm/xe: Convert pagefaulting code to use drm_exec")
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Cc: Matthew Brost <matthew.brost@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240208132115.3132-1-thomas.hellstrom@linux.intel.com
(cherry picked from commit eb538b55)
Signed-off-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>

drivers/gpu/drm/xe/xe_vm.c

@@ -995,9 +995,16 @@ int xe_vm_prepare_vma(struct drm_exec *exec, struct xe_vma *vma,
 	int err;
 
 	XE_WARN_ON(!vm);
+	if (num_shared)
 		err = drm_exec_prepare_obj(exec, xe_vm_obj(vm), num_shared);
-	if (!err && bo && !bo->vm)
+	else
+		err = drm_exec_lock_obj(exec, xe_vm_obj(vm));
+	if (!err && bo && !bo->vm) {
+		if (num_shared)
 			err = drm_exec_prepare_obj(exec, &bo->ttm.base, num_shared);
+		else
+			err = drm_exec_lock_obj(exec, &bo->ttm.base);
+	}
 
 	return err;
 }