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.

Fixes: ebd912ff ("s390/stacktrace: Merge perf_callchain_user() and arch_stack_walk_user()")
Cc: stable@vger.kernel.org
Reviewed-by: Jens Remus <jremus@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>