powerpc/sstep: Fix VSX instruction emulation

Commit af99da74 ("powerpc/sstep: Support VSX vector paired storage
access instructions") added loading and storing 32 word long data into
adjacent VSRs. However the calculation used to determine if two VSRs
needed to be loaded/stored inadvertently prevented the load/storing
taking place for instructions with a data length less than 16 words.

This causes the emulation to not function correctly, which can be seen
by the alignment_handler selftest:

$ ./alignment_handler
[snip]
test: test_alignment_handler_vsx_207
tags: git_version:powerpc-5.12-1-0-g82d2c16b
VSX: 2.07B
        Doing lxsspx:   PASSED
        Doing lxsiwax:  FAILED: Wrong Data
        Doing lxsiwzx:  PASSED
        Doing stxsspx:  PASSED
        Doing stxsiwx:  PASSED
failure: test_alignment_handler_vsx_207
test: test_alignment_handler_vsx_300
tags: git_version:powerpc-5.12-1-0-g82d2c16b
VSX: 3.00B
        Doing lxsd:     PASSED
        Doing lxsibzx:  PASSED
        Doing lxsihzx:  PASSED
        Doing lxssp:    FAILED: Wrong Data
        Doing lxv:      PASSED
        Doing lxvb16x:  PASSED
        Doing lxvh8x:   PASSED
        Doing lxvx:     PASSED
        Doing lxvwsx:   FAILED: Wrong Data
        Doing lxvl:     PASSED
        Doing lxvll:    PASSED
        Doing stxsd:    PASSED
        Doing stxsibx:  PASSED
        Doing stxsihx:  PASSED
        Doing stxssp:   PASSED
        Doing stxv:     PASSED
        Doing stxvb16x: PASSED
        Doing stxvh8x:  PASSED
        Doing stxvx:    PASSED
        Doing stxvl:    PASSED
        Doing stxvll:   PASSED
failure: test_alignment_handler_vsx_300
[snip]

Fix this by making sure all VSX instruction emulation correctly
load/store from the VSRs.

Fixes: af99da74 ("powerpc/sstep: Support VSX vector paired storage access instructions")
Signed-off-by: Jordan Niethe <jniethe5@gmail.com>
Reviewed-by: Ravi Bangoria <ravi.bangoria@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210225031946.1458206-1-jniethe5@gmail.com

arch/powerpc/lib/sstep.c

@@ -904,7 +904,7 @@ static nokprobe_inline int do_vsx_load(struct instruction_op *op,
 	if (!address_ok(regs, ea, size) || copy_mem_in(mem, ea, size, regs))
 		return -EFAULT;
 
-	nr_vsx_regs = size / sizeof(__vector128);
+	nr_vsx_regs = max(1ul, size / sizeof(__vector128));
 	emulate_vsx_load(op, buf, mem, cross_endian);
 	preempt_disable();
 	if (reg < 32) {
@@ -951,7 +951,7 @@ static nokprobe_inline int do_vsx_store(struct instruction_op *op,
 	if (!address_ok(regs, ea, size))
 		return -EFAULT;
 
-	nr_vsx_regs = size / sizeof(__vector128);
+	nr_vsx_regs = max(1ul, size / sizeof(__vector128));
 	preempt_disable();
 	if (reg < 32) {
 		/* FP regs + extensions */