s390: Fix off-by-one error for task size detection

The entries in /proc/<pid>/maps look like the following:

  3fffffdf000-40000000000 rw-p 00000000 00:00 0

The upper address is the first address that is *not* included in the
address range.

Our function max_mapped_addr() should return the last valid address
for a process, but currently returns the first invalid address.

This can lead to the following error message on kernel that have
kernel commit ee71d16d22bb:

 Error (criu/proc_parse.c:694): Can't dump high memory region
 1ffffffffff000-20000000000000 of task 24 because kernel commit ee71d16d22bb
 is missing

Fix this and make max_mapped_addr() the last valid address (first invalid
address - 1).

Reported-by: Adrian Reber <areber@redhat.com>
Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>