f2fs: avoid to drop nat entries due to the negative nr_shrink

The try_to_free_nats should not receive the negative nr_shrink.
Otherwise, it can drop all the nat entries by the while loop.
Signed-off-by: NJaegeuk Kim <jaegeuk.kim@samsung.com>

fs/f2fs/node.c

@@ -208,7 +208,7 @@ 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 <= NM_WOUT_THRESHOLD)
+	if (nm_i->nat_cnt <= NM_WOUT_THRESHOLD || nr_shrink <= 0)
 		return 0;
 
 	write_lock(&nm_i->nat_tree_lock);