dlm: check required context while close

This patch adds a WARN_ON() check to validate the right context while
dlm_midcomms_close() is called. Even before commit 489d8e55


("fs: dlm: add reliable connection if reconnect") in this context
dlm_lowcomms_close() flushes all ongoing transmission triggered by dlm
application stack. If we do that, it's required that no new message will
be triggered by the dlm application stack. The function
dlm_midcomms_close() is not called often so we can check if all
lockspaces are in such context.

Signed-off-by: Alexander Aring <aahringo@redhat.com>
Signed-off-by: David Teigland <teigland@redhat.com>