in_pcb: add in_pcbrele_rlock()

The helper that derefs and rlocks the provided inp.  Returns false if inp
is still usable.

Reviewed by:	glebius, markj
Sponsored by:	Nvidia networking
Differential revision:	https://reviews.freebsd.org/D51143

sys/netinet/in_pcb.c

@@ -1744,6 +1744,23 @@ in_pcbrele(struct inpcb *inp, const inp_lookup_t lock)
 	    in_pcbrele_rlocked(inp) : in_pcbrele_wlocked(inp));
 }
 
+/*
+ * Dereference and rlock inp, for which the caller must own the
+ * reference.  Returns true if inp no longer usable, false otherwise.
+ */
+bool
+in_pcbrele_rlock(struct inpcb *inp)
+{
+	INP_RLOCK(inp);
+	if (in_pcbrele_rlocked(inp))
+		return (true);
+	if ((inp->inp_flags & INP_FREED) != 0) {
+		INP_RUNLOCK(inp);
+		return (true);
+	}
+	return (false);
+}
+
 /*
  * Unconditionally schedule an inpcb to be freed by decrementing its
  * reference count, which should occur only after the inpcb has been detached

sys/netinet/in_pcb.h

@@ -681,6 +681,7 @@ void	in_pcbref(struct inpcb *);
 bool	in_pcbrele(struct inpcb *, inp_lookup_t);
 bool	in_pcbrele_rlocked(struct inpcb *);
 bool	in_pcbrele_wlocked(struct inpcb *);
+bool	in_pcbrele_rlock(struct inpcb *inp);
 
 typedef bool inp_match_t(const struct inpcb *, void *);
 struct inpcb_iterator {