This patch wraps scattered optional parameters into discard policy as
below, later, with it we expect that we can adjust these parameters with
proper strategy in different scenario.

struct discard_policy {
	unsigned int min_interval;	/* used for candidates exist */
	unsigned int max_interval;	/* used for candidates not exist */
	unsigned int max_requests;	/* # of discards issued per round */
	unsigned int io_aware_gran;	/* minimum granularity discard not be aware of I/O */
	bool io_aware;			/* issue discard in idle time */
	bool sync;			/* submit discard with REQ_SYNC flag */
};

This patch doesn't change any logic of codes.
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>