linuxkpi: Add refcount_dec_and_test

In Linux this takes a refcount_t argument but in linuxkpi struct kref
uses an atomic_t for the refcount and code in drm directly uses this
function with a kref so use an atomic_t here.

Reviewed by:	bz
Sponsored by:	Beckhoff Automation GmbH & Co. KG
Differential Revision:	https://reviews.freebsd.org/D36099

sys/compat/linuxkpi/common/include/linux/kref.h

@@ -43,6 +43,7 @@
 #include <asm/atomic.h>
 
 struct kref {
+	/* XXX In Linux this is a refcount_t */
 	atomic_t refcount;
 };
 

sys/compat/linuxkpi/common/include/linux/refcount.h

@@ -79,4 +79,15 @@ refcount_dec_and_lock_irqsave(refcount_t *ref, spinlock_t *lock,
 	return (false);
 }
 
+/*
+ * struct kref uses atomic_t and not refcount_t so
+ * we differ from Linux here.
+ */
+static inline bool
+refcount_dec_and_test(atomic_t *r)
+{
+
+	return (atomic_dec_and_test(r));
+}
+
 #endif /* __LINUXKPI_LINUX_REFCOUNT_H__ */