Revert "f2fs: enhance multi-threads performance"

This reports performance regression by Yuanhan Liu.
The basic idea was to reduce one-point mutex, but it turns out this causes
another contention like context swithes.

https://lkml.org/lkml/2015/4/21/11

Until finishing the analysis on this issue, I'd like to revert this for a while.

This reverts commit 78373b7319abdf15050af5b1632c4c8b8b398f33.

fs/f2fs/data.c

···
 {
 	struct inode *inode = mapping->host;
 	struct f2fs_sb_info *sbi = F2FS_I_SB(inode);
+	bool locked = false;
 	int ret;
 	long diff;
 
···
 
 	diff = nr_pages_to_write(sbi, DATA, wbc);
 
+	if (!S_ISDIR(inode->i_mode)) {
+		mutex_lock(&sbi->writepages);
+		locked = true;
+	}
 	ret = write_cache_pages(mapping, wbc, __f2fs_writepage, mapping);
+	if (locked)
+		mutex_unlock(&sbi->writepages);
 
 	f2fs_submit_merged_bio(sbi, DATA, WRITE);
 

fs/f2fs/f2fs.h

···
 	struct mutex cp_mutex;			/* checkpoint procedure lock */
 	struct rw_semaphore cp_rwsem;		/* blocking FS operations */
 	struct rw_semaphore node_write;		/* locking node writes */
+	struct mutex writepages;		/* mutex for writepages() */
 	wait_queue_head_t cp_wait;
 
 	struct inode_management im[MAX_INO_ENTRY];      /* manage inode cache */

fs/f2fs/super.c

···
 	sbi->raw_super = raw_super;
 	sbi->raw_super_buf = raw_super_buf;
 	mutex_init(&sbi->gc_mutex);
+	mutex_init(&sbi->writepages);
 	mutex_init(&sbi->cp_mutex);
 	init_rwsem(&sbi->node_write);
 	clear_sbi_flag(sbi, SBI_POR_DOING);