GFS2: Update handling of DLM return codes to match reality

GFS2's idea of which return codes it needs to handle was based
upon those listed in dlm.h. Those didn't cover all the possible
codes and listed some which never happen. This updates GFS2 to
handle all the codes which can actually be returned from the
DLM under various circumstances.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>