Another fix for r253823; retain the default of 1 readahead block for sendfile.

Submitted by:	glebius
Obtained from:	Netflix
MFC after:	3 days
Allocate in6_ifextra (ifp->if_afdata[AF_INET6]) only for IPv6-capable
interfaces.  This eliminates unnecessary IPv6 processing for non-IPv6
interfaces.

MFC after:	3 days