f2fs: Don't create discard thread when device doesn't support realtime discard

Don't create discard thread when device doesn't support realtime discard
or user specifies nodiscard mount option.

Signed-off-by: Fengnan Chang <changfengnan@vivo.com>
Signed-off-by: Yangtao Li <frank.li@vivo.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>