dlm: wait for outstanding SRCU callbacks to complete in exit paths

The dlm_lowcomms_exit() and dlm_midcomms_exit() iterate over the
srcu protected connection and node hash tables and hand each
element to call_srcu() for deferred freeing (connection_release()
and midcomms_node_release()). call_srcu() is asynchronous: the
callbacks are invoked only after an SRCU grace period, which may
happen after the exit function has already returned.

These exit functions are reached from exit_dlm() on module unload.
Once they return, module teardown continues and the module text
may be unloaded while call_srcu() callbacks are still pending. When
such a callback finally runs, it executes freed module code and
touches the static SRCU domains that are being torn down, resulting
in a use-after-free.

Add an srcu_barrier() after the call_srcu() loop in each exit function
to wait for all outstanding callbacks of the respective SRCU domain to
complete before returning. In dlm_midcomms_exit() the barrier is issued
before dlm_lowcomms_exit() so that node callbacks are drained prior to
tearing down the lower layer.

Signed-off-by: Zqiang <qiang.zhang@linux.dev>
Acked-by: Alexander Aring <aahringo@redhat.com>
Signed-off-by: Alexander Aring <aahringo@redhat.com>
Signed-off-by: David Teigland <teigland@redhat.com>