OSDMap: clarify is_{down,in}() definitions

Make it clear that is_down() == !is_up() and is_in() == !is_out().
This is not a functional change.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>
Merge pull request #1428 from ceph/wip-is_down-boolean

OSDMap: clarify is_{down,in}() definitions

Reviewed-by: Sage Weil <sage@inktank.com>