powerpc/memcpy: Fix stack corruption for smaller sizes

For sizes lesser than 128 bytes, the code branches out early without saving
the stack frame, which when restored later drops frame of the caller.
Tested-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Santosh Sivaraj <santosh@fossix.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190903214359.23887-1-santosh@fossix.org

arch/powerpc/lib/memcpy_mcsafe_64.S

@@ -84,7 +84,6 @@ err1;	stw	r0,0(r3)
 
 3:	sub	r5,r5,r6
 	cmpldi	r5,128
-	blt	5f
 
 	mflr	r0
 	stdu	r1,-STACKFRAMESIZE(r1)
@@ -99,6 +98,7 @@ err1;	stw	r0,0(r3)
 	std	r22,STK_REG(R22)(r1)
 	std	r0,STACKFRAMESIZE+16(r1)
 
+	blt	5f
 	srdi	r6,r5,7
 	mtctr	r6