block/genhd: use seq_put_decimal_ull for diskstats decimal values

seq_printf is costly. For each block device, 19 decimal values are
yielded in /proc/diskstats via seq_printf. On a system with 16 logical
block devices, profiling for open/read/close sequences shows seq_printf
took ~75% samples of diskstats_show:

	diskstats_show(92.626% 2269372/2450040)
	    seq_printf(76.026% 1725313/2269372)
		vsnprintf(99.163% 1710866/1725313)
		    format_decode(26.597% 455040/1710866)
		    number(19.554% 334542/1710866)
		    memcpy_orig(4.183% 71570/1710866)
			...
		srso_return_thunk(0.009% 148/1725313)
	    part_stat_read_all(8.030% 182236/2269372)

One million rounds of open/read/close /proc/diskstats takes:

	real	0m37.687s
	user	0m0.264s
	sys	0m32.911s
On average, each sequence tooks ~0.032ms

With this patch, most decimal values are yield via seq_put_decimal_ull,
performance is significantly improved:

	real	0m20.792s
	user	0m0.316s
	sys	0m20.463s
On average, each sequence tooks ~0.020ms, a ~37.5% improvement.

Signed-off-by: David Wang <00107082@163.com>
Link: https://lore.kernel.org/r/20241108054500.4251-1-00107082@163.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>