drm/i915/gem: Replace user_access_begin by user_write_access_begin

When i915_gem_execbuffer2_ioctl() is using user_access_begin(),
that's only to perform unsafe_put_user() so use
user_write_access_begin() in order to only open write access.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/ebf1250b6d4f351469fb339e5399d8b92aa8a1c1.1585898438.git.christophe.leroy@c-s.fr

drivers/gpu/drm/i915/gem/i915_gem_execbuffer.c

@@ -2794,7 +2794,8 @@ i915_gem_execbuffer2_ioctl(struct drm_device *dev, void *data,
 		 * And this range already got effectively checked earlier
 		 * when we did the "copy_from_user()" above.
 		 */
-		if (!user_access_begin(user_exec_list, count * sizeof(*user_exec_list)))
+		if (!user_write_access_begin(user_exec_list,
+					     count * sizeof(*user_exec_list)))
 			goto end;
 
 		for (i = 0; i < args->buffer_count; i++) {
@@ -2808,7 +2809,7 @@ i915_gem_execbuffer2_ioctl(struct drm_device *dev, void *data,
 					end_user);
 		}
 end_user:
-		user_access_end();
+		user_write_access_end();
 end:;
 	}