When adding the extra 10% of space to a needed sysctl buffer use math

When adding the extra 10% of space to a needed sysctl buffer use math
that is less likely to overflow the int type used. A BGP fullfeed is
now so big that this calculation overflowed and then got sign extended.
The result was for example 'route -n show' failures.
Problem identified with deraadt@
OK deraadt@ (more cleanup needed but this fix is a good start)