Retire vm_reserv_extend_{contig,page}(). These functions were introduced
as part of a false start toward fine-grained reservation locking. In the
end, they were not needed, so eliminate them.

Order the parameters to vm_reserv_alloc_{contig,page}() consistently with
the vm_page functions that call them.

Update the comments about the locking requirements for
vm_reserv_alloc_{contig,page}(). They no longer require a free page
queues lock.

Wrap several lines that became too long after the "req" and "domain"
parameters were added to vm_reserv_alloc_{contig,page}().

Reviewed by: kib, markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D20492