ext2/ext3/ext4: add block bitmap validation

When a new block bitmap is read from disk in read_block_bitmap() there are
a few bits that should ALWAYS be set.  In particular, the blocks given by
ext4_blk_bitmap, ext4_inode_bitmap and ext4_inode_table.  Validate the
block bitmap against these blocks.

[akpm@linux-foundation.org: cleanups]
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Andreas Dilger <adilger@clusterfs.com>
Acked-by: Mingming Cao <cmm@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>