fs: dlm: fix race in mhandle deletion

This patch fixes a race between mhandle deletion in case of receiving an
acknowledge and flush of all pending mhandle in cases of an timeout or
resetting node states.

Fixes: 489d8e55 ("fs: dlm: add reliable connection if reconnect")
Reported-by: Guillaume Nault <gnault@redhat.com>
Signed-off-by: Alexander Aring <aahringo@redhat.com>
Signed-off-by: David Teigland <teigland@redhat.com>