s390/stacktrace: Use break instead of return statement

In the Linux kernel, the following vulnerability has been resolved: s390/stacktrace: Use break instead of return statement arch_stack_walk_user_common() contains a return statement instead of a break statement in case store_ip() fails while trying to store a callchain entry of a user space process. This may lead to a missing pagefault_enable() call. If this happens any subsequent page fault of the process won't be resolved by the page fault handler and this in turn will lead to the process being killed. Use a break instead of a return statement to fix this.