f2fs: fix wrong pointer access during try_to_free_nids

If we release the lock in list_for_each_entry_safe, we can lose the tmp
pointer by alloc_nid.

Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/node.c

diff --git a/fs/f2fs/node.c b/fs/f2fs/node.c
index 777066d..0867325 100644
--- a/fs/f2fs/node.c
+++ b/fs/f2fs/node.c
@@ -1664,11 +1664,9 @@
 		if (i->state == NID_ALLOC)
 			continue;
 		__del_from_free_nid_list(nm_i, i);
-		nm_i->fcnt--;
-		spin_unlock(&nm_i->free_nid_list_lock);
 		kmem_cache_free(free_nid_slab, i);
+		nm_i->fcnt--;
 		nr_shrink--;
-		spin_lock(&nm_i->free_nid_list_lock);
 	}
 	spin_unlock(&nm_i->free_nid_list_lock);
 	mutex_unlock(&nm_i->build_lock);