fs: dlm: replace use of socket sk_callback_lock with sock_lock

This patch will replace the use of socket sk_callback_lock lock and uses
socket lock instead. Some users like sunrpc, see commit ea9afca8
("SUNRPC: Replace use of socket sk_callback_lock with sock_lock") moving
from sk_callback_lock to sock_lock which seems to be held when the socket
callbacks are called.

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