[IPV6] ADDRCONF: Clean-up ipv6_dev_get_saddr().

old:
|    text	   data	    bss	    dec	    hex	filename
|   28599	   1416	     96	  30111	   759f	net/ipv6/addrconf.o

new:
|    text	   data	    bss	    dec	    hex	filename
|   28007	   1416	     96	  29519	   734f	net/ipv6/addrconf.o

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