f2fs: control not to exceed # of cached nat entries

This is to avoid cache entry management overhead including radix tree.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
diff --git a/fs/f2fs/node.c b/fs/f2fs/node.c
index 16532b3..b448c8f 100644
--- a/fs/f2fs/node.c
+++ b/fs/f2fs/node.c
@@ -52,6 +52,10 @@
 		mem_size = (nm_i->nat_cnt * sizeof(struct nat_entry)) >>
 							PAGE_SHIFT;
 		res = mem_size < ((avail_ram * nm_i->ram_thresh / 100) >> 2);
+		if (excess_cached_nats(sbi))
+			res = false;
+		if (nm_i->nat_cnt > DEF_NAT_CACHE_THRESHOLD)
+			res = false;
 	} else if (type == DIRTY_DENTS) {
 		if (sbi->sb->s_bdi->wb.dirty_exceeded)
 			return false;