libceph: use for_each_request() in ceph_osdc_abort_on_full()
Scanning the trees just to see if there is anything to abort is
unnecessary -- all that is needed here is to update the epoch barrier
first, before we start aborting. Simplify and do the update inside the
loop before calling abort_request() for the first time.
The switch to for_each_request() also fixes a bug: homeless requests
weren't even considered for aborting.