osd: add 'osd debug reject backfill probability' option

This will make the OSD randomly reject backfill reservation requests.  This
exercises the failure code paths but does not break overall behavior
because the primary will back off and retry later.

This should help us reproduce #7922.

Signed-off-by: Sage Weil <sage@inktank.com>
osd: Send REJECT to all previously acquired reservations

When getting a REJECT from a backfill target, tell already GRANTed targets to
go back to RepNotRecovering state by sending a REJECT to them.

Fixes: #7922

Signed-off-by: David Zafman <david.zafman@inktank.com>
Merge pull request #1600 from ceph/wip-7922

Wip 7922

Passes my manual testing and the new teuthology test case.

Reviewed-by: Sage Weil <sage@inktank.com>