mon: fix `osd crush link` id resolution

A junk value for ID was falling through from
the outer scope.  Rename the outer one to
`osdid` and look up `id` explicitly in crush link.

For a bonus also add an error string for when
link_bucket returns an error (previously
would get plain "Error EINVAL")

Signed-off-by: John Spray <john.spray@redhat.com>
Merge pull request #2808 from ceph/wip-crush-link-id

mon: fix `osd crush link` id resolution

Reviewed-by: Sage Weil <sage@redhat.com>