uapi linux/dlm_netlink.h: include linux/dlmconstants.h

Fixes userspace compilation error:

error: ‘DLM_RESNAME_MAXLEN’ undeclared here (not in a function)
  char resource_name[DLM_RESNAME_MAXLEN];

Signed-off-by: Mikko Rapeli <mikko.rapeli@iki.fi>
Signed-off-by: David Teigland <teigland@redhat.com>