osd: clear need_up_thru in build_prior as appropriate

osd: clear need_up_thru in build_prior as appropriate

The only time need_up_thru is cleared is in the Peering state AdvMap
handler, but it doesn't get called if prior_set_affected() and we go
into build_prior().  Build_prior() sets need_up_thru if it's needed, but
it doesn't clear it if its not, which means the pg never goes active.

Reported-by: Sam Lang <samlang@gmail.com>
Signed-off-by: Sage Weil <sage@newdream.net>