libceph: don't abort reads in ceph_osdc_abort_on_full()
Don't consider reads for aborting and use ->base_oloc instead of
->target_oloc, as done in __submit_request().
Strictly speaking, we shouldn't be aborting FULL_TRY/FULL_FORCE writes
either. But, there is an inconsistency in FULL_TRY/FULL_FORCE handling
on the OSD side [1], so given that neither of these is used in the
kernel client, leave it for when the OSD behaviour is sorted out.