Adjust filt_solisten for poll and select

[Visa Hankala]

The listen version of EVFILT_READ for sockets differs somewhat from
the code in soo_poll(). The following patch adds a poll/select branch
to make the listen filter more consistent with the old code.

OK?

Index: kern/uipc_socket.c
===================================================================
RCS file: src/sys/kern/uipc_socket.c,v
retrieving revision 1.270
diff -u -p -r1.270 uipc_socket.c
--- kern/uipc_socket.c  13 Dec 2021 14:56:55 -0000      1.270
+++ kern/uipc_socket.c  23 Dec 2021 14:20:54 -0000
@@ -2318,11 +2318,23 @@ filt_soexceptprocess(struct knote *kn, s
 int
 filt_solisten_common(struct knote *kn, struct socket *so)
 {
+       int active;
+
        soassertlocked(so);
 
        kn->kn_data = so->so_qlen;
+       active = (kn->kn_data != 0);
+
+       if (kn->kn_flags & (__EV_POLL | __EV_SELECT)) {
+               if (so->so_state & SS_ISDISCONNECTED) {
+                       kn->kn_flags |= __EV_HUP;
+                       active = 1;
+               } else {
+                       active = soreadable(so);
+               }
+       }
 
-       return (kn->kn_data != 0);
+       return (active);
 }
 
 int