drm/i915: vfree() no longer ignores the low bits of the address

Since vfree() now likes to WARN when passed a non-page-aligned pointer,
we need to discard the low bits to comply with it.

Fixes: d31d7cb1460c ("drm/i915: Support for creating write combined type vmaps")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20160818161718.27187-3-chris@chris-wilson.co.uk
diff --git a/drivers/gpu/drm/i915/i915_gem.c b/drivers/gpu/drm/i915/i915_gem.c
index a8d0f70..cffa57b 100644
--- a/drivers/gpu/drm/i915/i915_gem.c
+++ b/drivers/gpu/drm/i915/i915_gem.c
@@ -2094,11 +2094,14 @@
 	list_del(&obj->global_list);
 
 	if (obj->mapping) {
-		/* low bits are ignored by is_vmalloc_addr and kmap_to_page */
-		if (is_vmalloc_addr(obj->mapping))
-			vunmap(obj->mapping);
+		void *ptr;
+
+		ptr = ptr_mask_bits(obj->mapping);
+		if (is_vmalloc_addr(ptr))
+			vunmap(ptr);
 		else
-			kunmap(kmap_to_page(obj->mapping));
+			kunmap(kmap_to_page(ptr));
+
 		obj->mapping = NULL;
 	}