librbd: account m_processing when failing request after refresh

librbd: account m_processing when failing request after refresh

AioImageRequestWQ::_void_process() dequeues an item from the
work queue by invoking ThreadPool::_void_process() which also
performs accounting by incrementing ->m_processing which never
gets decremented if an image refresh fails. This is exactly
the flow when running test_lock_fence.sh which blacklists a
client and expects failed I/O operation from the client.

Without this fix, running test_lock_fence.sh in a loop fails
with an assert in PointerWQ dtor within 100 odd runs.

Fixes: http://tracker.ceph.com/issues/17973
Signed-off-by: Venky Shankar <vshankar@redhat.com>