drm/i915: Don't prefault the entire obj if the vma is smaller

Take the minimum of the object size and the vma size and prefault
only that much. Avoids a SIGBUS when mmapping only a portion of the
object.

Prefaulting was introduced here:
 commit b90b91d87038f6b257b40a02b42ed4f9705e06f0
 Author: Chris Wilson <chris@chris-wilson.co.uk>
 Date:   Tue Jun 10 12:14:40 2014 +0100

    drm/i915: Prefault the entire object on first page fault

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Testcase: igt/gem_mmap/short-mmap
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/i915_gem.c

@@ -1570,9 +1570,12 @@ int i915_gem_fault(struct vm_area_struct *vma, struct vm_fault *vmf)
 	pfn >>= PAGE_SHIFT;
 
 	if (!obj->fault_mappable) {
+		unsigned long size = min_t(unsigned long,
+					   vma->vm_end - vma->vm_start,
+					   obj->base.size);
 		int i;
 
-		for (i = 0; i < obj->base.size >> PAGE_SHIFT; i++) {
+		for (i = 0; i < size >> PAGE_SHIFT; i++) {
 			ret = vm_insert_pfn(vma,
 					    (unsigned long)vma->vm_start + i * PAGE_SIZE,
 					    pfn + i);