bootmem: reorder interface functions and add a missing one

Impact: cleanup and addition of missing interface wrapper

The interface functions in bootmem.h was ordered in not so orderly
manner.  Reorder them such that

* functions allocating the same area group together -
  ie. alloc_bootmem group and alloc_bootmem_low group.

* functions w/o node parameter come before the ones w/ node parameter.

* nopanic variants are immediately below their panicky counterparts.

While at it, add alloc_bootmem_pages_node_nopanic() which was missing.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Johannes Weiner <hannes@saeurebad.de>