vm_pageout: shrink pageout array

The array passed to vm_pageout_flush, and constructed in a middle-out
fashion, can never use array element zero. Shrink the array by one,
and reduce indices by one, to save that bit of stack space. In the
vm_object version, make the accounting look more like the pageout
version.

Reported by: alc
Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D46208