ext4: promote ext4 over ext2 in the default probe order

Prevent clean ext3 filesystems from mounting by default with the ext2
driver (with no journal!) by putting ext4 ahead of ext2 in the default
probe order.  This will have the effect of mounting ext2 filesystems
with ext4.ko by default, which is a safer failure than hoping the user
notices that their journalled ext3 is now running without a journal!

Users who require ext2.ko for ext2 can either disable ext4.ko or
explicitly request ext2 via "mount -t ext2" or "rootfstype=ext2".

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>