PG::actingset should be used when checking the number of acting OSDs for a given PG.
Signed-off-by: Guang Yang <yguang@yahoo-inc.com>
Merge pull request #2616 from guangyy/wip-giant-9614

PG::actingset should be used when checking the number of acting OSDs for a given PG.

Backport: firefly
Reviewed-by: Samuel Just <sam.just@inktank.com>