s390/dumpstack: resolve userspace last_break

report_user_fault() currently does not show which library last_break
points to. Call print_vma_addr() to find out; the output now looks
like this:

    Last Breaking-Event-Address:
     [<000003ffaa2a56e4>] libc.so.6[3ffaa180000+251000]

For kernel it's unchanged:

    Last Breaking-Event-Address:
     [<000000000030fd06>] trace_hardirqs_on+0x56/0xc8

Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Acked-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>