kernel - Rearrange pmap_growkernel() and fix up x86_64's pmap_growkernel()

kernel - Rearrange pmap_growkernel() and fix up x86_64's pmap_growkernel()

* Rearrange pmap_growkernel() to pass an address range instead of just an
  end address.

* vm_map() no longer retries after calling pmap_growkernel(), the function
  either succeeds or panics.

* x86_64 - kldloading modules after system boot no longer populates 128GB
  worth of page tables (which eats ~256MB of ram) in order to push
  kernel_vm_end past KERNBASE.

  Instead, any kldloads into the space beyond KERNBASE, which is required
  to be able to link them into the kernel due to 32-bit PC-relative
  addressing, will populate just the required page tables and will not
  bump up kernel_vm_end.

* Fixes a panic which can occur well after boot when kldload'ing the first
  module, if 256MB worth of free pages is not instantly available.

* Saves us 256M of ram on x86_64.