dlm: remove lock_sock to avoid scheduling while atomic

Before this patch, functions save_callbacks and restore_callbacks
called function lock_sock and release_sock to prevent other processes
from messing with the struct sock while the callbacks were saved and
restored. However, function add_sock calls write_lock_bh prior to
calling it save_callbacks, which disables preempts. So the call to
lock_sock would try to schedule when we can't schedule.

Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: David Teigland <teigland@redhat.com>