sys/netinet6/in6_pcb.c: fix compile without INET
    
    in6_mapped_sockaddr() and in6_mapped_peeraddr() both define a local
    variable named 'inp', but in the non-INET case, this variable is set
    and never used, causing a compiler error:
    
    /src/freebsd/src/lf/sys/netinet6/in6_pcb.c:547:16: error:
    	variable 'inp' set but not used [-Werror,-Wunused-but-set-variable]
      547 |         struct  inpcb *inp;
          |                        ^
    /src/freebsd/src/lf/sys/netinet6/in6_pcb.c:573:16: error:
    	variable 'inp' set but not used [-Werror,-Wunused-but-set-variable]
      573 |         struct  inpcb *inp;
    
    Fix this by guarding all the INET-specific logic, including the variable
    definition, behind #ifdef INET.
    
    While here, tweak formatting in in6_mapped_peeraddr() so both functions
    are the same.
    
    Reviewed by: imp
    Pull Request: https://github.com/freebsd/freebsd-src/pull/1155
    
    (cherry picked from commit 042fb58d009e7efc5b334b68fffbef9b1f620ec8)