f2fs: update cur_valid_map_mir together with cur_valid_map

When cur_valid_map passes the f2fs_test_and_set(,clear)_bit test,
cur_valid_map_mir update is skipped unlikely, so fix it. The fix
now changes the mirror check together with cur_valid_map all the
time.

Signed-off-by: Yunlong Song <yunlong.song@huawei.com>
Signed-off-by: Chao Yu <yuchao0@huawei.com>
[Jaegeuk Kim: Fix unused variable and add unlikely for corner condition.]
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>