f2fs: fix performance issue observed with multi-thread sequential read

This reverts the commit - "b93f7712 - f2fs: remove writepages lock"
to fix the drop in sequential read throughput.

Test: ./tiotest -t 32 -d /data/tio_tmp -f 32 -b 524288 -k 1 -k 3 -L
device: UFS

Before -
read throughput: 185 MB/s
total read requests: 85177 (of these ~80000 are 4KB size requests).
total write requests: 2546 (of these ~2208 requests are written in 512KB).

After -
read throughput: 758 MB/s
total read requests: 2417 (of these ~2042 are 512KB reads).
total write requests: 2701 (of these ~2034 requests are written in 512KB).

Change-Id: I8200ac7207f17a70aa2b4b7efcfeb6c5c6ad12be
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs.git


Git-commit: 853137cef46ccc490e6fd4b160a1c252d6459842
[stummala@codeaurora.org: Resolved merge conflicts]
Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>