msg: fix entity_addr_t::is_same_host() for IPv6

We weren't checking the memcmp return value properly!  Aie...

Backport: bobtail
Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit c8dd2b67b39a8c70e48441ecd1a5cc3c6200ae97)