After rhel6 patch landed, that's what you get when trying to use it on centos5 in dmesg:
ldiskfs created from ext4-2.6-rhel5
fsfilt_ldiskfs: Unknown symbol ldiskfs_discard_preallocations

It seems the discard prealloc code checks for the function availability, but not whenever it is exported, so we either need to adjust the heck or add another patch to centos5.