f2fs: fix out-of-order execution in f2fs_issue_flush

commit 6f890df0a7efe3181aceb5d8bcd4af7deb2abce5 upstream.

In f2fs_issue_flush, due to out-of-order execution of CPU, wake_up can
be called before we insert issue_list, result in long latency of
wait_for_completion. Fix this by adding smp_mb() to force the order of
related codes.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Git-commit: 6ed7ad60
Git-repo: https://android.googlesource.com/kernel/common.git


Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>