f2fs: optimize fs_lock for better performance
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: Jaegeuk Kim <20424effba9d4aeeb410130c5f23aa328fa29e09@samsung.com>
Signed-off-by: Yu Chao <0cc5db7488e13b393f245aa6263acec5e92fdcc5@samsung.com>
Signed-off-by: Gu Zheng <9eae2d327ddc4c2ff66f290d0aa6288f14fc5928@cn.fujitsu.com>
Signed-off-by: Jaegeuk Kim <20424effba9d4aeeb410130c5f23aa328fa29e09@samsung.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs