sanlock: add lvb feature
    
    set_lvb and get_lvb allow up to a sector size of
    application specific data to be stored with a
    resource lease.  They are similar to the lvb
    feature found in the dlm.
    
    host 1:
    sanlock_acquire(ACQUIRE_LVB, r)
    application data written to lvb
    sanlock_set_lvb(r, lvb)
    sanlock_release(r)
    
    The lvb data is copied to the sanlock daemon in set_lvb,
    and written to the resource lease on disk in release.
    
    host 2:
    sanlock_acquire(ACQUIRE_LVB, r)
    sanlock_get_lvb(r, lvb)
    application data read from lvb
    sanlock_release(r)
    
    The lvb data is read by the sanlock daemon from disk
    in acquire, and copied to the application in get_lvb.
    
    Signed-off-by: David Teigland <teigland@redhat.com>