f2fs: make sure trace all f2fs_issue_flush

The root device's issue flush trace is missing,
add it and tracing the result from submit.

Fixes d50aaeec ("f2fs: show actual device info in tracepoints")
Signed-off-by: Kinglong Mee <kinglongmee@gmail.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>