Add TASKQID_INVALID

Add the TASKQID_INVALID macros and update callers to use the macro
instead of testing against 0.  There is no functional change
even though the functions in zfs_ctldir.c incorrectly used -1
instead of 0.

Reviewed-by: Tom Caputi <tcaputi@datto.com>
Reviewed-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #5347

include/sys/zfs_context.h

@@ -495,6 +495,8 @@ typedef struct taskq {
 #define	TQ_NOQUEUE	0x02		/* Do not enqueue if can't dispatch */
 #define	TQ_FRONT	0x08		/* Queue in front */
 
+#define	TASKQID_INVALID		((taskqid_t)0)
+
 extern taskq_t *system_taskq;
 
 extern taskq_t	*taskq_create(const char *, int, pri_t, int, int, uint_t);