Small rework of txg_list code

This patch simply adds some missing locking to the txg_list
functions and refactors txg_verify() so that it is only compiled
in for debug builds.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
Closes #7795

include/sys/txg.h

@@ -133,6 +133,13 @@ extern void *txg_list_next(txg_list_t *tl, void *p, uint64_t txg);
 /* Global tuning */
 extern int zfs_txg_timeout;
 
+
+#ifdef ZFS_DEBUG
+#define	TXG_VERIFY(spa, txg)		txg_verify(spa, txg)
+#else
+#define	TXG_VERIFY(spa, txg)
+#endif
+
 #ifdef	__cplusplus
 }
 #endif