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 78373b7319.

fs/f2fs/f2fs.h

@@ -625,6 +625,7 @@ struct f2fs_sb_info {
 	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 */