osd/ReplicatedPG: handle dup ops earlier in do_op

osd/ReplicatedPG: handle dup ops earlier in do_op

Current the dup op checks happen in execute_ctx, long after we handle
cache ops or get the obc and (potentially) return ENOENT.  That means that
object deletions and cache ops both aren't properly idempotent.

This is easy to fix by moving the check earlier in do_op.

Fixes: #8089
Signed-off-by: Sage Weil <sage@inktank.com>