f2fs: fix to cover __allocate_new_section() with curseg_lock

In order to avoid race with f2fs_do_replace_block().

Fixes: f5a53edc ("f2fs: support aligned pinned file")
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>