This patch enables reading node blocks in advance when truncating large
data blocks.

 > time rm $MNT/testfile (500GB) after drop_cachees
Before : 9.422 s
After  : 4.821 s
Reported-by: NStephen Bates <stephen.bates@microsemi.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>