Rename clear_page_tables to clear_page_range. clear_page_range takes byte
ranges, and aggressively frees page table pages. Maybe useful to control
page table memory consumption on 4-level architectures (and even 3 level
ones).

Possible downsides are:
- flush_tlb_pgtables gets called more often (only a problem for sparc64
  AFAIKS).

- the opportunistic "expand to fill PGDIR_SIZE hole" logic that ensures
  something actually gets done under the old system is still in place.
  This could sometimes make unmapping small regions more inefficient. There
  are some other solutions to look at if this is the case though.
Signed-off-by: Nick Piggin <nickpiggin@yahoo.com.au>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>