powerpc/sstep: Fix array out of bound warning

Compiling kernel with -Warray-bounds throws below warning:

  In function 'emulate_vsx_store':
  warning: array subscript is above array bounds [-Warray-bounds]
  buf.d[2] = byterev_8(reg->d[1]);
  ~~~~~^~~
  buf.d[3] = byterev_8(reg->d[0]);
  ~~~~~^~~

Fix it by using temporary array variable 'union vsx_reg buf32[]' in
that code block. Also, with element_size = 32, 'union vsx_reg *reg'
is an array of size 2. So, use 'reg' as an array instead of pointer
in the same code block.

Fixes: af99da74 ("powerpc/sstep: Support VSX vector paired storage access instructions")
Suggested-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Signed-off-by: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Tested-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210129071745.111466-1-ravi.bangoria@linux.ibm.com

arch/powerpc/lib/sstep.c

@@ -818,13 +818,15 @@ void emulate_vsx_store(struct instruction_op *op, const union vsx_reg *reg,
 			break;
 		if (rev) {
 			/* reverse 32 bytes */
-			buf.d[0] = byterev_8(reg->d[3]);
-			buf.d[1] = byterev_8(reg->d[2]);
-			buf.d[2] = byterev_8(reg->d[1]);
-			buf.d[3] = byterev_8(reg->d[0]);
-			reg = &buf;
-		}
+			union vsx_reg buf32[2];
+			buf32[0].d[0] = byterev_8(reg[1].d[1]);
+			buf32[0].d[1] = byterev_8(reg[1].d[0]);
+			buf32[1].d[0] = byterev_8(reg[0].d[1]);
+			buf32[1].d[1] = byterev_8(reg[0].d[0]);
+			memcpy(mem, buf32, size);
+		} else {
 			memcpy(mem, reg, size);
+		}
 		break;
 	case 16:
 		/* stxv, stxvx, stxvl, stxvll */