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

This reverts the commit - "b93f771 - 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).

Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/f2fs.h

@@ -913,6 +913,7 @@ struct f2fs_sm_info {
 	unsigned int ipu_policy;	/* in-place-update policy */
 	unsigned int min_ipu_util;	/* in-place-update threshold */
 	unsigned int min_fsync_blocks;	/* threshold for fsync */
+	unsigned int min_seq_blocks;	/* threshold for sequential blocks */
 	unsigned int min_hot_blocks;	/* threshold for hot block allocation */
 	unsigned int min_ssr_sections;	/* threshold to trigger SSR allocation */
 
@@ -1133,6 +1134,7 @@ struct f2fs_sb_info {
 	struct rw_semaphore sb_lock;		/* lock for raw super block */
 	int valid_super_block;			/* valid super block no */
 	unsigned long s_flag;				/* flags for sbi */
+	struct mutex writepages;		/* mutex for writepages() */
 
 #ifdef CONFIG_BLK_DEV_ZONED
 	unsigned int blocks_per_blkz;		/* F2FS blocks per zone */