xpmem: fix mapping of attachment and segment

* Mapping attached part of segment is done at attach time instead of
  make time to work with runtimes (e.g. OpenMPI) xpmem_make-ing the
  entire user-space
* Mapping attached part of segment at attach time can be turned off by
  specifying xpmem_remote_on_demand in kernel argument
* Mapping attachment chooses appropriate page-sizes, i.e., largest
  allowed by memory range and segment page boundary

Fixes: a8696d8 "xpmem: Support large page attachment"
Change-Id: I44663865204036520e5f62fe22b9134ee4629f9b

kernel/process.c

@@ -1926,7 +1926,9 @@ out:
 	return error;
 }
 
-static int page_fault_process_memory_range(struct process_vm *vm, struct vm_range *range, uintptr_t fault_addr, uint64_t reason)
+int page_fault_process_memory_range(struct process_vm *vm,
+				    struct vm_range *range,
+				    uintptr_t fault_addr, uint64_t reason)
 {
 	int error;
 	pte_t *ptep;