f2fs: delete DIO read lock

This lock can be a contention with multi 4k random read IO with single inode.

example) fio --output=test --name=test --numjobs=60 --filename=/media/samsung960pro/file_test --rw=randread --bs=4k
 --direct=1 --time_based --runtime=7 --ioengine=libaio --iodepth=256 --group_reporting --size=10G

With this commit, it remove that possible lock contention.

Signed-off-by: Dongjoo Seo <commisori28@gmail.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>