[XFRM] IPV6: Optimize __xfrm_tunnel_alloc_spi().

| % size old/net/ipv6/xfrm6_tunnel.o new/net/ipv6/xfrm6_tunnel.o
|    text	   data	    bss	    dec	    hex	filename
|    1606	     40	   2080	   3726	    e8e	old/net/ipv6/xfrm6_tunnel.o
|    1574	     40	   2080	   3694	    e6e	new/net/ipv6/xfrm6_tunnel.o

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