f2fs: use lock-less list(llist) to simplify the flush cmd management

We use flush cmd control to collect many flush cmds, and flush them
together. In this case, we use two list to manage the flush cmds
(collect and dispatch), and one spin lock is used to protect this.
In fact, the lock-less list(llist) is very suitable to this case,
and we use simplify this routine.

-
v2:
-use llist_for_each_entry_safe to fix possible use-after-free issue.
-remove the unused field from struct flush_cmd.
Thanks for Yu's suggestion.
-

Signed-off-by: Gu Zheng <guz.fnst@cn.fujitsu.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>