vm_pageout: simplify pageout_cluster

Rewrite vm_pageout_cluster to eliminate redundant variables and
duplicated code.

Remove tests on pindex to check for object boundary conditions, since
the page_next and page_prev functions return NULL at the object
boundaries. Fix an alignment error that could happen if pindex is
aligned, and the first of vm_pageout_page_count flushable pages, and
the page at pindex-1 is also flushable.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D46217