f2fs: make __queue_discard_cmd() return void

Since __queue_discard_cmd() never returns an error,
let's make it return void.

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