gfs_controld: read plocks from dlm or lock_dlm
    
    bz 450169
    
    For bug 450138, the kernel plock code was moved from gfs2 into the dlm.
    gfs_controld needs to be adapted to this change, and read plocks from
    either lock_dlm (kernels before 5.3), or dlm (kernels 5.3 and later).
    
    The misc device gfs_controld reads/writes has previously been called
    "lock_dlm_plock", but the patch for bug 450138 changes the name of
    the device to "dlm_plock".  gfs_controld looks for "dlm_plock", and
    if it's not found, looks for "lock_dlm_plock".  If "lock_dlm_plock"
    is found, it knows that the filesystem/lockspace id in the plock
    structures needs to be translated.  Otherwise, the operation of
    plocks remains the same.
    
    Signed-off-by: David Teigland <teigland@redhat.com>