linuxkpi: Add __var_waitqueue

This returns the wait queue based on the object but in LinuxKPI
we only have one waitqueue for this.

Reviewed by:	hselasky, bz
MFC after:	2 weeks
Sponsored by:	Beckhoff Automation GmbH & Co. KG
Differential Revision:	https://reviews.freebsd.org/D34250

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

@@ -64,4 +64,10 @@ wake_up_var(void *var)
 	wake_up(&linux_var_waitq);
 }
 
+static inline wait_queue_head_t *
+__var_waitqueue(void *p)
+{
+	return (&linux_var_waitq);
+}
+
 #endif	/* __LINUXKPI_LINUX_WAITBIT_H__ */