apply_to_page_range() takes an address range, and if any parts of it are
not covered by the existing page table hierarchy, it allocates memory to
fill them in.

In some use cases, this is not what we want - we want to be able to
operate exclusively on PTEs that are already in the tables.

Add apply_to_existing_page_range() for this.  Adjust the walker
functions for apply_to_page_range to take 'create', which switches them
between the old and new modes.

This will be used in KASAN vmalloc.

[akpm@linux-foundation.org: reduce code duplication]
[akpm@linux-foundation.org: s/apply_to_existing_pages/apply_to_existing_page_range/]
[akpm@linux-foundation.org: initialize __apply_to_page_range::err]
Link: http://lkml.kernel.org/r/20191205140407.1874-1-dja@axtens.netSigned-off-by: Daniel Axtens <dja@axtens.net>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Uladzislau Rezki (Sony) <urezki@gmail.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Daniel Axtens <dja@axtens.net>
Cc: Qian Cai <cai@lca.pw>
Cc: Andrey Ryabinin <aryabinin@virtuozzo.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>