f2fs: keep scanning until enough free nids are acquired

In current version, after scan_free_nid_bits, the scan is over if
nid_cnt[FREE_NID] != 0. In most cases, there are still free nids in the
free list during the scan, and scan_free_nid_bits usually can't increase
nid_cnt[FREE_NID]. It causes that __build_free_nids is called many times
without solving the shortage of the free nids. This patch fixes that.

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