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>

fs/f2fs/node.c

@@ -2015,7 +2015,7 @@ static void __build_free_nids(struct f2fs_sb_info *sbi, bool sync, bool mount)
 		/* try to find free nids in free_nid_bitmap */
 		scan_free_nid_bits(sbi);
 
-		if (nm_i->nid_cnt[FREE_NID])
+		if (nm_i->nid_cnt[FREE_NID] >= NAT_ENTRY_PER_BLOCK)
 			return;
 	}