dlm: fix creating multiple node structures

This patch will lookup existing nodes instead of always creating them
when dlm_midcomms_addr() is called. The idea is here to create midcomms
nodes when user space getting informed that nodes joins the cluster. This
is the case when dlm_midcomms_addr() is called, however it can be called
multiple times by user space to add several address configurations to one
node e.g. when using SCTP. Those multiple times need to be filtered out
and we doing that by looking up if the node exists before. Due configfs
entry it is safe that this function gets only called once at a time.

Fixes: 63e711b0 ("fs: dlm: create midcomms nodes when configure")
Signed-off-by: Alexander Aring <aahringo@redhat.com>
Signed-off-by: David Teigland <teigland@redhat.com>