ext4: Use an rbtree for tracking blocks freed during transaction.

(cherry picked from commit c894058d66637c7720569fbe12957f4de64d9991 to allow
commit e21675d4 to be included in 2.6.27.y)

With this patch we track the block freed during a transaction using
red-black tree.  We also make sure contiguous blocks freed are collected
in one node in the tree.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>