acquire inp lock around ip6_pcbopt to fix IPV6_TCLASS panic

[jason_eggnet.com]

Summary

Hopefully simple fix to address panics relating to setting IPV6_TCLASS with setsockopt().

The premise of this fix is that it is ok to call malloc with M_NOWAIT while holding a lock on the in6p.

If it is not ok, then major surgery will be required, as ip6_setpktopt() will have to be fixed (as it also calls malloc with M_NOWAIT) which pulls in the ip6_pcbopts(), ip6_setpktopts(), ip6_setpktopt() call chain. The other call chain does have to be fixed but it can be done in a different review (and with relative ease) if this strategy works out.

[jason_eggnet.com]

use correct constants