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.