Improve bufdone_finish()'s handling of the bogus page.  Specifically, if
one or more mappings to the bogus page must be replaced, call pmap_qenter()
just once.  Previously, pmap_qenter() was called for each mapping to the
bogus page.

Change the implementation of vm_hold_free_pages() so that it performs at
most one call to pmap_qremove(), and thus one TLB shootdown, instead of one
call and TLB shootdown per page.

Simplify the interface to vm_hold_free_pages().