f2fs: better to wait for fstrim completion

commit 1eb1ef4a8e9f6a4d9c2c7a645aba21d2f8719728 upstream.

In android, we'd better wait for fstrim completion instead of issuing the
discard commands asynchronous.

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