kasan: simplify address description logic

kasan: simplify address description logic

Simplify logic for describing a memory address.  Add addr_to_page()
helper function.

Makes the code easier to follow.

Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Andrey Konovalov <[email protected]>
Acked-by: Dmitry Vyukov <[email protected]>
Cc: Andrey Ryabinin <[email protected]>
Cc: Alexander Potapenko <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>