f2fs: ro: don't start discard thread for readonly image

[ 9299.893835] F2FS-fs (vdd): Allow to mount readonly mode only
mount: /mnt/f2fs: WARNING: source write-protected, mounted read-only.
root@qemu:/ ps -ef|grep f2fs
root          94       2  0 03:46 ?        00:00:00 [kworker/u17:0-f2fs_post_read_wq]
root        6282       2  0 06:21 ?        00:00:00 [f2fs_discard-253:48]

There will be no deletion in readonly image, let's skip starting
discard thread to save system resources.

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