f2fs: add a function to move nid

This patch add a new function to move nid from one state to another.
Move operation is heavily used, by adding a new function for it
we can cut down some branches from several flow.

Signed-off-by: Fan li <fanofcode.li@samsung.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>