osd/PeeringState: transit async_recovery_targets back into acting before backfilling

osd/PeeringState: transit async_recovery_targets back into acting before backfilling

When an osd that is part of current up set gets chosen as an
async_recovery_target, it gets removed from the acting set.
Since we don't allow any want that is larger than the pool size,
a pg must transit into UNDERSIZED when asynchronous recovery
eventually happens.
However, if that pg has one or more backfill targets, it might
spin UNDERSIZED for a long time during which mon will keep issuing
"PG_AVAILABILITY" warns until all backfill targets finally completes.

Fix by calling choose_acting to get any async_recovery_targets back
into acting before we continue to backfill.

Fixes: https://tracker.ceph.com/issues/43311
Signed-off-by: xie xingguo <xie.xingguo@zte.com.cn>
(cherry picked from commit 48bc4786fd73b538b673e6a7eb7ced986d95005f)

Conflicts:
	src/osd/PeeringState.cc
- file does not exist in nautilus; backported the code change manually
  to src/osd/PG.cc