Fix: fencer: avoid use-after-free with self-fencing and topology

In the case of self-fencing with topology, handle_fence_request() will
overwrite F_STONITH_OPERATION in the original request XML, which invalidates
the request.op pointer created by stonith_command(). The fix is to make
request.op a copy.

Regression introduced in 2.1.3 by 067d655eb