nd6: properly fill CARP link level address for NS sent out

[glebius]

Summary

When sending an NS, check if we are using a CARP address and
if we do, then add proper address into ND_OPT_SOURCE_LINKADDR
option and also put PACKET_TAG_CARP tag on the packet. The
code really follows what NA sending code has been doing since
introduction of carp(4). While here, bring to style(9) the
whole block of code.

PR: 193280