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>

arch/openrisc/kernel/head.S

@@ -1069,8 +1069,7 @@ d_pte_not_present:
 	EXCEPTION_LOAD_GPR4
 	EXCEPTION_LOAD_GPR5
 	EXCEPTION_LOAD_GPR6
-	l.j	_dispatch_do_dpage_fault
-	l.nop
+	EXCEPTION_HANDLE(_dtlb_miss_page_fault_handler)
 
 /* ==============================================[ ITLB miss handler ]=== */
 ENTRY(itlb_miss_handler)
@@ -1192,8 +1191,7 @@ i_pte_not_present:
 	EXCEPTION_LOAD_GPR4
 	EXCEPTION_LOAD_GPR5
 	EXCEPTION_LOAD_GPR6
-	l.j	_dispatch_do_ipage_fault
-	l.nop
+	EXCEPTION_HANDLE(_itlb_miss_page_fault_handler)
 
 /* ==============================================[ boot tlb handlers ]=== */