f2fs: add mount option for segment allocation policy

This patch adds an mount option, "alloc_mode=%s" having two options, "default"
and "reuse".

In "alloc_mode=reuse" case, f2fs starts to allocate segments from 0'th segment
all the time to reassign segments. It'd be useful for small-sized eMMC parts.

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