ipcs: make shmall overflow a bit less worse

ipcs: make shmall overflow a bit less worse

Still no large integer support but on overflow we print now
the largest possible value, maybe even the largest one which
makes sense at all.

So on x86_64 systems we'll see now:
$ echo "4503599627370496" > /proc/sys/kernel/shmall
$ ipcs -m -l | grep "max total"
max total shared memory (kbytes) = 18014398509481980

rather than this:
$ ipcs -m -l | grep "max total"
max total shared memory (kbytes) = 0

Signed-off-by: Ruediger Meier <ruediger.meier@ga-group.nl>