ipv6: Optmize translation between IPV6_PREFER_SRC_xxx and RT6_LOOKUP_F_xxx.

IPV6_PREFER_SRC_xxx definitions:
| #define IPV6_PREFER_SRC_TMP             0x0001
| #define IPV6_PREFER_SRC_PUBLIC          0x0002
| #define IPV6_PREFER_SRC_COA             0x0004

RT6_LOOKUP_F_xxx definitions:
| #define RT6_LOOKUP_F_SRCPREF_TMP        0x00000008
| #define RT6_LOOKUP_F_SRCPREF_PUBLIC     0x00000010
| #define RT6_LOOKUP_F_SRCPREF_COA        0x00000020

So, we can translate between these two groups by shift operation
instead of multiple 'if's.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>