IB/addr: Correct CONFIG_IPv6 to CONFIG_IPV6

Correct misspelled "CONFIG_IPv6" that was introduced in commit
d14714df ("IB/addr: Fix IPv6 routing lookup").  The config variable
should be all uppercase.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
[ This was my fault when I munged the original patch.  - Roland ]

Signed-off-by: Roland Dreier <rolandd@cisco.com>