f2fs: split discard_cmd_list

Split discard_cmd_list to discard_{pend,wait}_list, so while sending/waiting
discard command, we can avoid traversing unneeded entries in original list.

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