ext4: Convert list_for_each_rcu() to list_for_each_entry_rcu()

The list_for_each_entry_rcu() primitive should be used instead of
list_for_each_rcu(), as the former is easier to use and provides
better type safety.

http://groups.google.com/group/linux.kernel/browse_thread/thread/45749c83451cebeb/0633a65759ce7713?lnk=raot

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Roel Kluin <12o3l@tiscali.nl>
Signed-off-by: Mingming Cao <cmm@us.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>