Refactor: fencer: functionize common part of timeout handling

Previously, remote_op_timeout() was called from multiple places, but only one
of those places needed the full processing. The common part is now in a new
function finalize_timed_out_op() called from all the places, and
remote_op_timeout() now has just the additional processing needed by the one
place plus a call to the new function.

This will allow a future change to set a different exit reason depending on
which step timed out.