f2fs: remove struct victim_selection default_v_ops

There is only single instance of these ops, and Jaegeuk point out that:

    Originally this was intended to give a chance to provide other
    allocation option. Anyway, it seems quit hard to do it anymore.

So remove the indirection and call f2fs_get_victim() directly.

Signed-off-by: Yangtao Li <frank.li@vivo.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>