fs: dlm: bring back previous shutdown handling

This patch mostly reverts commit 4f567acb ("fs: dlm: remove socket
shutdown handling"). There can be situations where the dlm midcomms nodes
hash and lowcomms connection hash are not equal, but we need to guarantee
that the lowcomms are all closed on a last release of a dlm lockspace,
when a shutdown is invoked. This patch guarantees that we always close
all sockets managed by the lowcomms connection hash, and calls shutdown
for the last message sent. This ensures we don't cut the socket, which
could cause the peer to get a connection reset.

In future we should try to merge the midcomms/lowcomms hashes into one
hash and not handle both in separate hashes.

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