fs: dlm: fix proper srcu api call

This patch will use call_srcu() instead of call_rcu() because the
related datastructure resource are handled under srcu context. I assume
the current code is fine anyway since free_conn() must be called when
the related resource are not in use otherwise. However it will correct
the overall handling in a srcu context.

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