f2fs: control the memory footprint used by ino entries

This patch adds to control the memory footprint used by ino entries.
This will conduct best effort, not strictly.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
index 16721b5d..e094675 100644
--- a/fs/f2fs/segment.c
+++ b/fs/f2fs/segment.c
@@ -276,7 +276,8 @@
 {
 	/* check the # of cached NAT entries and prefree segments */
 	if (try_to_free_nats(sbi, NAT_ENTRY_PER_BLOCK) ||
-				excess_prefree_segs(sbi))
+			excess_prefree_segs(sbi) ||
+			available_free_memory(sbi, INO_ENTRIES))
 		f2fs_sync_fs(sbi->sb, true);
 }