top shows 99% interrupt in -current

Hi,

Jason suggested that this change might fix a lockup issue I've been 
seeing, so I updated. top now shows 100% (well, 99% most of the time) 
interrupt, even though sysstat definitely shows normal interrupt activity 
and the system performs fine anyway, so this appears to be cosmetic.

> Modified Files:
> 	src/sys/arch/alpha/common: sgmap_typedep.c
>
> Log Message:
> Fix inverted logic!!
>
> vmem_xalloc(9) should be used with VM_NOSLEEP for BUS_DMA_NOWAIT.

If not this change, it would've had to have been something since 
2-April-2021.

Thanks,
John