f2fs: do not set free of current section

For the case when sbi->segs_per_sec > 1, take section:segment = 5 for
example, if segment 1 is just used and allocate new segment 2, and the
blocks of segment 1 is invalidated, at this time, the previous code will
use __set_test_and_free to free the free_secmap and free_sections++,
this is not correct since it is still a current section, so fix it.

Signed-off-by: Yunlong Song <yunlong.song@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>