Fstrim intends to trim invalid blocks of filesystem only with specified
range and granularity, but actually, it will issue all previous cached
discard commands which may be out-of-range and be with unmatched
granularity, it's unneeded.

In order to fix above issues, this patch introduces new helps to support
to issue and wait discard in range and adds a new fstrim_list for tracking
in-flight discard from ->fstrim.
Signed-off-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>