[XFRM] IPV6: Optimize xfrm6_input_addr().

| % size old/net/ipv6/xfrm6_input.o new/net/ipv6/xfrm6_input.o
|    text	   data	    bss	    dec	    hex	filename
|    1026	      0	      0	   1026	    402	old/net/ipv6/xfrm6_input.o
|     947	      0	      0	    947	    3b3	new/net/ipv6/xfrm6_input.o

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