wcfs: Add caller to WatchLink

A watch link is a link between WCFS and a specific client. Before this
patch, watch links didn't memorize basic information of the client (pid
and owner), now this information is added. This can be practical in case
the server needs to send signals to the client process. It's needed in
order to implement protection against faulty clients [1].

[1] https://lab.nexedi.com/nexedi/wendelin.core/blob/38dde766/wcfs/wcfs.go#L186-208

wcfs/wcfs.go

@@ -674,6 +674,8 @@ type WatchLink struct {
 	txMu    sync.Mutex
 	rxMu    sync.Mutex
 	rxTab   map[/*stream*/uint64]chan string // client replies go via here
+
+	caller fuse.Caller // client that opened the WatchLink
 }
 
 // Watch represents watching for changes to 1 BigFile over particular watch link.
@@ -1489,6 +1491,7 @@ func (w *Watch) _pin(ctx context.Context, blk int64, rev zodb.Tid) (err error) {
 		return err
 	}
 
+
 	if ack != "ack" {
 		blkpin.err = fmt.Errorf("expect %q; got %q", "ack", ack)
 		return blkpin.err
@@ -1814,6 +1817,7 @@ func (wnode *WatchNode) Open(flags uint32, fctx *fuse.Context) (nodefs.File, fus
 		head:   head,
 		byfile: make(map[zodb.Oid]*Watch),
 		rxTab:  make(map[uint64]chan string),
+		caller: fctx.Caller,
 	}
 
 	head.wlinkMu.Lock()