f2fs: make get_lock_data_page to handle encrypted inode

This patch refactors get_lock_data_page() to handle encryption case directly.
In order to do that, it introduces common f2fs_submit_page_read().

Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>