f2fs: optimize fs_lock for better performance

There is a performance problem: when all sbi->fs_lock are holded, then
all the following threads may get the same next_lock value from sbi->next_lock_num
in function mutex_lock_op, and wait for the same lock(fs_lock[next_lock]),
it may cause performance reduce.
So we move the sbi->next_lock_num++ before getting lock, this will average the
following threads if all sbi->fs_lock are holded.

v1-->v2:
	Drop the needless spin_lock as Jaegeuk suggested.
Suggested-by: NJaegeuk Kim <jaegeuk.kim@samsung.com>
Signed-off-by: NYu Chao <chao2.yu@samsung.com>
Signed-off-by: NGu Zheng <guz.fnst@cn.fujitsu.com>
Signed-off-by: NJaegeuk Kim <jaegeuk.kim@samsung.com>

fs/f2fs/f2fs.h

@@ -544,15 +544,15 @@ static inline void mutex_unlock_all(struct f2fs_sb_info *sbi)
 
 static inline int mutex_lock_op(struct f2fs_sb_info *sbi)
 {
-	unsigned char next_lock = sbi->next_lock_num % NR_GLOBAL_LOCKS;
+	unsigned char next_lock;
 	int i = 0;
 
 	for (; i < NR_GLOBAL_LOCKS; i++)
 		if (mutex_trylock(&sbi->fs_lock[i]))
 			return i;
 
+	next_lock = sbi->next_lock_num++ % NR_GLOBAL_LOCKS;
 	mutex_lock(&sbi->fs_lock[next_lock]);
-	sbi->next_lock_num++;
 	return next_lock;
 }