powerpc: rearrange do_page_fault error case to be inside exception_enter

This keeps the context tracking over the entire interrupt handler which
helps later with moving context tracking into interrupt wrappers.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210130130852.2952424-14-npiggin@gmail.com

arch/powerpc/mm/fault.c

@@ -545,19 +545,24 @@ NOKPROBE_SYMBOL(__do_page_fault);
 long do_page_fault(struct pt_regs *regs)
 {
 	const struct exception_table_entry *entry;
-	enum ctx_state prev_state = exception_enter();
-	int rc = __do_page_fault(regs, regs->dar, regs->dsisr);
-	exception_exit(prev_state);
-	if (likely(!rc))
-		return 0;
+	enum ctx_state prev_state;
+	long err;
 
-	entry = search_exception_tables(regs->nip);
-	if (unlikely(!entry))
-		return rc;
+	prev_state = exception_enter();
+	err = __do_page_fault(regs, regs->dar, regs->dsisr);
+	if (likely(!err))
+		goto out;
 
+	entry = search_exception_tables(regs->nip);
+	if (likely(entry)) {
 		instruction_pointer_set(regs, extable_fixup(entry));
+		err = 0;
+	}
 
-	return 0;
+out:
+	exception_exit(prev_state);
+
+	return err;
 }
 NOKPROBE_SYMBOL(do_page_fault);