objecter: move map checks to helper

objecter: move map checks to helper

This makes coverity happier because we check_op_pool_dne() may free
the Op (or Lingerop) structure(s), but the callers in the submit_*
paths dereference after calling.  This is actually safe because they
never free new ops, but is confusing.  Explicitly push this into a
separate helper.

CID 739607 (#1-2 of 2): Read from pointer after free (USE_AFTER_FREE)
At (9): Dereferencing freed pointer "o".

CID 739606 (#1 of 1): Read from pointer after free (USE_AFTER_FREE)
At (28): Dereferencing freed pointer "op".

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