s390/simd: clear vector register pointer on fork/clone

The copy_thread function fails to reset the p->thread.vxrs pointer.
This causes the child to use the same vector register save area,
causing both data corruptions and multiple frees of the memory for
the save area after the tasks sharing the save area terminate.
Signed-off-by: NMartin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/kernel/process.c

@@ -153,6 +153,7 @@ int copy_thread(unsigned long clone_flags, unsigned long new_stackp,
 	save_fp_ctl(&p->thread.fp_regs.fpc);
 	save_fp_regs(p->thread.fp_regs.fprs);
 	p->thread.fp_regs.pad = 0;
+	p->thread.vxrs = NULL;
 	/* Set a new TLS ?  */
 	if (clone_flags & CLONE_SETTLS) {
 		unsigned long tls = frame->childregs.gprs[6];