f2fs: fix iostat related lock protection

Made iostat related locks safe to be called from irq context again.

Cc: <stable@vger.kernel.org>
Fixes: a1e09b03 ("f2fs: use iomap for direct I/O")
Signed-off-by: Daeho Jeong <daehojeong@google.com>
Reviewed-by: Stanley Chu <stanley.chu@mediatek.com>
Tested-by: Eddie Huang <eddie.huang@mediatek.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>