ext2: disallow setting xip on remount

Yan Zheng pointed out that ext2_remount lacks checking if -o xip should be
enabled or not.  This patch checks for presence of direct_access on the
backing block device and if the blocksize meets the requirements.

Signed-off-by: Carsten Otte <cotte@de.ibm.com>
Cc: Yan Zheng <yanzheng@21cn.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>