inet6: nd6_na_input() now considers ln_state <= ND6_LLINFO_INCOMPLETE

Otherwise if ln_state != ND6_LLINFO_INCOMPLETE and the is no lladdr
and this message was solicited then ln_state is set to ND6_LLINFO_REACHABLE
which could then cause a panic in nd6_resolve().
If ln_state > ND6_LLINFO_INCOMPLETE then it's assumed we have a lladdr.

Potentially this could have been triggered by the introduction of
ND6_LLINFO_PURGE in nd6.c r1.143 but also by the re-introduction of
ND6_LLINFO_INCOMPLETE in nd6.c r1.263.
Depending on the timing, it's technically possible to receive such
a message after the llentry is created with ND6_LLINFO_NOSTATE.

(roy)