drm/i915/gtt: Fix the boundary check for vm area

The check for start + length >= total_vm_size is
wrong since start + length can be exactly the size of
the vm.

Fix the check to allow allocation to boundary.

Fixes a regression in commit 4dd738e9cd79
("drm/i915: Fix 32b overflow check in gen8_ppgtt_alloc_page_directories")

Testcase: igt/gem_evict_everything/swapping-interruptible
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=90399
Tested-by: Lu Hua <huax.lu@intel.com>
Cc: Chris Wilson <chris@chris.wilson.co.uk>
Cc: Dave Gordon <david.s.gordon@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: Michel Thierry <michel.thierry@intel.com>
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem_gtt.c

@@ -757,7 +757,7 @@ static int gen8_ppgtt_alloc_page_directories(struct i915_hw_ppgtt *ppgtt,
 	WARN_ON(!bitmap_empty(new_pds, GEN8_LEGACY_PDPES));
 
 	/* FIXME: upper bound must not overflow 32 bits  */
-	WARN_ON((start + length) >= (1ULL << 32));
+	WARN_ON((start + length) > (1ULL << 32));
 
 	gen8_for_each_pdpe(pd, pdp, start, length, temp, pdpe) {
 		if (pd)