mmu: pass type of a page fault (r/w) to a page allocator

Page allocation may behave differently depending on a type of a fault.
Read fault may return ARC page, but write may do cow for instance.

Reviewed-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Gleb Natapov <gleb@cloudius-systems.com>