ocfs2: Sync inode flags with ext2.

We sync our inode flags with ext2 and define them by hex
values. But actually in commit 36695673

(4 years ago), all
these values are moved to include/linux/fs.h. So we'd
better also use them as what ext2 did. So sync our inode
flags with ext2 by using FS_*.

Signed-off-by: Tao Ma <tao.ma@oracle.com>
Signed-off-by: Joel Becker <joel.becker@oracle.com>