sysctl_{tcp,udp,sctp}_mem overflow on 16TB system.

[Robin Holt <holt@sgi.com>]

On a 16TB system, we noticed that sysctl_tcp_mem[2] and sysctl_udp_mem[2]
were negative.  Code review indicates that the same should occur with
sysctl_sctp_mem[2].

There are a couple ways we could address this.  The one which appears most
reasonable would be to change the struct proto defintion for sysctl_mem
from an int to a long and handle all the associated fallout.

An alternative is to limit the calculation to 1/2 INT_MAX.  The downside
being that the administrator could not tune the system to use more than
INT_MAX memory when much more is available.

Is there a compelling reason to not change the structure's definition
over to longs instead of ints and deal with the fallout from that change?

Thanks,
Robin Holt