vm_pageout: reduce number of flush() params

vm_pageout_flush is called in two places, and in both, the mreq
parameter is 0. Drop that parameter, and simplify the calculations
that use it.

The prunlen and eio parameters are either both NULL, or neither
NULL. Drop the prunlen parameter and, when eio is NULL, return the
runlen value instead of the numpagedout parameter, which the caller
ignores.

Change a param from boolean_t* to bool*.

Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D50568