ext4: Use single thread to perform DIO unwritten convertion

While running ext4 testing on multiple core, we found there are per
cpu ext4-dio-unwritten threads processing conversion from unwritten
extents to written for IOs completed from async direct IO patch.  Per
filesystem is enough, we don't need per cpu threads to work on
conversion.

Signed-off-by: Mingming Cao <cmm@us.ibm.com>