powerpc/mm: Pre-filter SRR1 bits before do_page_fault()

By filtering the relevant SRR1 bits in the assembly rather than
in do_page_fault() itself, we avoid a conditional branch (since we
already come from different path for data and instruction faults).

This will allow more simplifications later

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/head_8xx.S

@@ -569,8 +569,8 @@ _ENTRY(DTLBMiss_jmp)
 InstructionTLBError:
 	EXCEPTION_PROLOG
 	mr	r4,r12
-	mr	r5,r9
-	andis.	r10,r5,0x4000
+	andis.	r5,r9,0x4820		/* Filter relevant SRR1 bits */
+	andis.	r10,r9,0x4000
 	beq+	1f
 	tlbie	r4
 itlbie: