openrisc: fix up vmalloc page table loading

vmalloc'ed pages are faulted into a process' page tables on demand.  In
order to facilitate this, do_page_fault needs to know whether it was
called via a page fault exception or a TLB-miss exception.

This patch adds a wrapper around the _x_page_fault_handler entry points
that the TLB-miss exceptions can call into in order to have the relevant
parameter set to satisfy do_page_fault.

This fixes a bug and is "good enough" for now.  That said, this whole
handling of vmalloc needs to be audited for correctness at some point.

Signed-off-by: Jonas Bonn <jonas@southpole.se>