powerpc: Fix parameter clobber in csum_partial_copy_generic()

The csum_partial_copy_generic() uses register r7 to adjust the remaining
bytes to process.  Unfortunately, r7 also holds a parameter, namely the
address of the flag to set in case of access exceptions while reading
the source buffer.  Lacking a quantum implementation of PowerPC, this
commit instead uses register r9 to do the adjusting, leaving r7's
pointer uncorrupted.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Anton Blanchard <anton@samba.org>
Cc: stable@vger.kernel.org
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/lib/checksum_64.S

@@ -269,8 +269,8 @@ _GLOBAL(csum_partial_copy_generic)
 	rldicl. r6,r3,64-1,64-2		/* r6 = (r3 & 0x3) >> 1 */
 	beq	.Lcopy_aligned
 
-	li	r7,4
-	sub	r6,r7,r6
+	li	r9,4
+	sub	r6,r9,r6
 	mtctr	r6
 
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