ext4: Calculate and verify checksums for htree nodes

Calculate and verify the checksum for directory index tree (htree)
node blocks.  The checksum is stored in the last 4 bytes of the htree
block and requires the dx_entry array to stop 1 dx_entry short of the
end of the block.

Signed-off-by: Darrick J. Wong <djwong@us.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>