[mips] [vm] restore translation of freelist to flind for page allocation

Commit r326346 moved domain iterators from physical layer to vm_page one,
but it also removed translation of freelist to flind for
vm_page_alloc_freelist() call. Before it expects VM_FREELIST_ parameter,
but after it expect freelist index.

On small WiFi boxes with few megabytes of RAM, there is only one freelist
VM_FREELIST_LOWMEM (1) and there is no VM_FREELIST_DEFAULT(0) (see file
sys/mips/include/vmparam.h). It results in freelist 1 with flind 0.

At first, this commit renames flind to freelist in vm_page_alloc_freelist
to avoid misunderstanding about input parameters. Then on physical layer it
restores translation for correct handling of freelist parameter.

Reported by: landonf
Reviewed by: jeff
Differential Revision: https://reviews.freebsd.org/D13351