f2fs: reduce region of cp_rwsem covered in f2fs_do_collapse

In f2fs_do_collapse, region cp_rwsem covered is large, since it will be
held until all blocks are left shifted, so if we try to collapse small
area at the beginning of large file, checkpoint who want to grab writer's
lock of cp_rwsem will be delayed for long time.

In order to avoid this condition, altering to lock/unlock cp_rwsem each
shift operation.

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>