Three changes to pmap_enter():

1. Use _pmap_alloc_l3() instead of pmap_alloc_l3() in order to handle the

possibility that a superpage mapping for "va" was created while we slept.
(This is derived from the amd64 version.)

2. Eliminate code for allocating kernel page table pages. Kernel page

table pages are preallocated by pmap_growkernel().

3. Eliminate duplicated unlock operations when KERN_RESOURCE_SHORTAGE is

returned.

MFC after: 2 weeks