powerpc: Fix string instr. emulation for 32-bit processes on ppc64

String instruction emulation would erroneously result in a segfault if
the upper bits of the EA are set and is so high that it fails access
check.  Truncate the EA to 32 bits if the process is 32-bit.
Signed-off-by: James Yang <James.Yang@freescale.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/traps.c

@@ -866,6 +866,10 @@ static int emulate_string_inst(struct pt_regs *regs, u32 instword)
 		u8 val;
 		u32 shift = 8 * (3 - (pos & 0x3));
 
+		/* if process is 32-bit, clear upper 32 bits of EA */
+		if ((regs->msr & MSR_64BIT) == 0)
+			EA &= 0xFFFFFFFF;
+
 		switch ((instword & PPC_INST_STRING_MASK)) {
 			case PPC_INST_LSWX:
 			case PPC_INST_LSWI: