f2fs: don't let the orphan inode counter underflow

Accounting errors from buggy code calling the acquire/release/remove
orphan inode interfaces can cause n_orphans to underflow, which will
then cause acquire_orphan_inode() to return -ENOSPC on the next
operation.  This commit guards against that condition.
Signed-off-by: Russ Knize <rknize@motorola.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>

fs/f2fs/checkpoint.c

@@ -206,6 +206,7 @@ int acquire_orphan_inode(struct f2fs_sb_info *sbi)
 void release_orphan_inode(struct f2fs_sb_info *sbi)
 {
 	mutex_lock(&sbi->orphan_inode_mutex);
+	BUG_ON(sbi->n_orphans == 0);
 	sbi->n_orphans--;
 	mutex_unlock(&sbi->orphan_inode_mutex);
 }
@@ -253,6 +254,7 @@ void remove_orphan_inode(struct f2fs_sb_info *sbi, nid_t ino)
 		if (orphan->ino == ino) {
 			list_del(&orphan->list);
 			kmem_cache_free(orphan_entry_slab, orphan);
+			BUG_ON(sbi->n_orphans == 0);
 			sbi->n_orphans--;
 			break;
 		}