f2fs: shrink free_nids entries

This patch introduces __count_free_nids/try_to_free_nids and registers
them in slab shrinker for shrinking under memory pressure.

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/segment.c

diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
index bf1605d..1b42656 100644
--- a/fs/f2fs/segment.c
+++ b/fs/f2fs/segment.c
@@ -310,6 +310,9 @@
 	if (!available_free_memory(sbi, NAT_ENTRIES))
 		try_to_free_nats(sbi, NAT_ENTRY_PER_BLOCK);
 
+	if (!available_free_memory(sbi, FREE_NIDS))
+		try_to_free_nids(sbi, NAT_ENTRY_PER_BLOCK * FREE_NID_PAGES);
+
 	/* checkpoint is the only way to shrink partial cached entries */
 	if (!available_free_memory(sbi, NAT_ENTRIES) ||
 			excess_prefree_segs(sbi) ||