Fix style issues, a typo in "kern.ipc.nmbufs" and correctly plave and
expose the value of the tunable maxmbufmem as "kern.ipc.maxmbufmem"
through sysctl.

Reported by:	smh
MFC after:	1 day
Make use of the fact that uma_zone_set_max(9) already returns the
rounded limit making a call to uma_zone_get_max(9) unnecessary.

MFC after:	1 day