nfsd: rename delegation related tracepoints to make them less confusing

Now when a read delegation is given, two delegation related traces
will be printed:

    nfsd_deleg_open: client 5f45b854:e6058001 stateid 00000030:00000001
    nfsd_deleg_none: client 5f45b854:e6058001 stateid 0000002f:00000001

Although the intention is to let developers know two stateid are
returned, the traces are confusing about whether or not a read delegation
is handled out. So renaming trace_nfsd_deleg_none() to trace_nfsd_open()
and trace_nfsd_deleg_open() to trace_nfsd_deleg_read() to make
the intension clearer.

The patched traces will be:

    nfsd_deleg_read: client 5f48a967:b55b21cd stateid 00000003:00000001
    nfsd_open: client 5f48a967:b55b21cd stateid 00000002:00000001

Suggested-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>