inetd: only declare and use the mapped-address netconfig under INET6

In a WITHOUT_INET6 build the only assignment to netid2 is compiled out and
the non-INET6 arm returns early, so netid2 is unconditionally NULL and the
rpcb_set() call guarded by it is dead code. clang does not prove it dead
and reports nbuf2 as uninitialized where it is passed as a const pointer,

No functional change.

MFC after: 1 week
Reported by: clang (-Wuninitialized-const-pointer)
Suggested by: dim
Approved by: ngie (co-mentor)

Reviewed by: ngie
Differential Revision: https://reviews.freebsd.org/D59277