f2fs: fix inconsistent using of NM_WOUT_THRESHOLD

try_to_free_nats() is usually called with parameter nr_shrink as
	"nm_i->nat_cnt - NM_WOUT_THRESHOLD"
by flush_nat_entries() during checkpointing process.

However, this is inconsistent with the actual threshold check as
	"if (nm_i->nat_cnt < 2 * NM_WOUT_THRESHOLD)"
, which will ignore the free_nats requests when
	NM_WOUT_THRESHOLD < nm_i->nat_cnt < 2 * NM_WOUT_THRESHOLD

So fix the threshold check condition.
Signed-off-by: NHaicheng Li <haicheng.li@linux.intel.com>
Signed-off-by: NJaegeuk Kim <jaegeuk.kim@samsung.com>

fs/f2fs/node.c

@@ -244,7 +244,7 @@ static int try_to_free_nats(struct f2fs_sb_info *sbi, int nr_shrink)
 {
 	struct f2fs_nm_info *nm_i = NM_I(sbi);
 
-	if (nm_i->nat_cnt < 2 * NM_WOUT_THRESHOLD)
+	if (nm_i->nat_cnt <= NM_WOUT_THRESHOLD)
 		return 0;
 
 	write_lock(&nm_i->nat_tree_lock);