[IPV6] ADDRLABEL: Sparse: Make several functions static.

Fix following sparse warnings:
| net/ipv6/addrlabel.c:172:25: warning: symbol 'ip6addrlbl_alloc' was not declared. Should it be static?
| net/ipv6/addrlabel.c:219:5: warning: symbol '__ip6addrlbl_add' was not declared. Should it be static?
| net/ipv6/addrlabel.c:260:5: warning: symbol 'ip6addrlbl_add' was not declared. Should it be static?
| net/ipv6/addrlabel.c:285:5: warning: symbol '__ip6addrlbl_del' was not declared. Should it be static?
| net/ipv6/addrlabel.c:311:5: warning: symbol 'ip6addrlbl_del' was not declared. Should it be static?

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