f2fs: don't hold cmd_lock during waiting discard command

Previously, with protection of cmd_lock, we will wait for end io of
discard command which potentially may lead long latency, making worse
concurrency.

So, in this patch, we try to add reference into discard entry to prevent
the entry being released by other thread, then we can avoid holding
global cmd_lock during waiting discard to finish.

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