[GFS2] use pid for plock owner for nfs clients

The fl_owner is that of lockd when posix locks arrive from nfs
clients, so it can't be used to distinguish between lock holders.
Use fl_pid as owner instead; it's the pid of the process on the
nfs client.

Signed-off-by: David Teigland <teigland@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>