f2fs: fix writing incorrect orphan blocks

Previously, there was a erroneous scenario like below.
thread 1:                       thread 2:
 f2fs_unlink
  - acquire_orphan_inode
    : sbi->n_orphans++           write_checkpoint
                                 - block_operations
                                  : f2fs_lock_all
                                 - do_checkpoint
                                  : write orphan blocks with sbi->n_orphans
                                 - unblock_operations
  - f2fs_lock_op
  - release_orphan_inode
  - f2fs_unlock_op

During the checkpoint by thread 2, f2fs stores a wrong orphan block according
to the wrong sbi->n_orphans.
To avoid this, simply we should make cover acquire_orphan_inode too with
f2fs_lock_op.
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>

fs/f2fs/namei.c

@@ -228,14 +228,14 @@ static int f2fs_unlink(struct inode *dir, struct dentry *dentry)
 	if (!de)
 		goto fail;
 
+	f2fs_lock_op(sbi);
 	err = acquire_orphan_inode(sbi);
 	if (err) {
+		f2fs_unlock_op(sbi);
 		kunmap(page);
 		f2fs_put_page(page, 0);
 		goto fail;
 	}
-
-	f2fs_lock_op(sbi);
 	f2fs_delete_entry(de, page, inode);
 	f2fs_unlock_op(sbi);