f2fs: use rb-tree to track pending discard commands

Introduce rb-tree based discard cache infrastructure to speed up lookup and
merge operation of discard entry.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
[Jaegeuk Kim: initialize dc to avoid build warning]
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>