dlm: Split lock mode and flag constants into a sharable header.

This allows others to use the DLM constants without being tied to the
function API of fs/dlm.

Signed-off-by: Joel Becker <joel.becker@oracle.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>
Signed-off-by: David Teigland <teigland@redhat.com>
Signed-off-by: Mark Fasheh <mark.fasheh@oracle.com>