[IPV6] NDISC: Unify main process of sending ND messages.

Because ndisc_send_na(), ndisc_send_ns() and ndisc_send_rs()
are almost identical, so let's unify their common part.

With gcc (GCC) 3.3.5 (Debian 1:3.3.5-13) on i386,
	Before:
	   text	   data	    bss	    dec	    hex	filename
	  14689	    364	     24	  15077	   3ae5	net/ipv6/ndisc.o
	After:
	   text	   data	    bss	    dec	    hex	filename
	  12317	    364	     24	  12705	   31a1	net/ipv6/ndisc.o

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>