f2fs: shrink nat_cache entries

This patch registers shrinking nat_cache entries.

Reviewed-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
index 61b97f9..d5ee992 100644
--- a/fs/f2fs/segment.c
+++ b/fs/f2fs/segment.c
@@ -306,8 +306,12 @@
 	/* try to shrink extent cache when there is no enough memory */
 	f2fs_shrink_extent_tree(sbi, EXTENT_CACHE_SHRINK_NUMBER);
 
-	/* check the # of cached NAT entries and prefree segments */
-	if (try_to_free_nats(sbi, NAT_ENTRY_PER_BLOCK) ||
+	/* check the # of cached NAT entries */
+	if (!available_free_memory(sbi, NAT_ENTRIES))
+		try_to_free_nats(sbi, NAT_ENTRY_PER_BLOCK);
+
+	/* checkpoint is the only way to shrink partial cached entries */
+	if (!available_free_memory(sbi, NAT_ENTRIES) ||
 			excess_prefree_segs(sbi) ||
 			!available_free_memory(sbi, INO_ENTRIES))
 		f2fs_sync_fs(sbi->sb, true);