dsl_dir_tempreserve_impl: remove unused deferred variable

The following commit moved the users of `deferred` into function
dsl_pool_unreserved_space:

    commit d2734cc
    Author: Serapheim Dimitropoulos <serapheim.dimitro@delphix.com>
    Date:   Fri Dec 16 14:11:29 2016 -0800

        OpenZFS 9166 - zfs storage pool checkpoint

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <freqlabs@FreeBSD.org>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Christian Schwarz <christian.schwarz@nutanix.com>
Closes openzfs#13056

module/zfs/dsl_dir.c

@@ -1320,7 +1320,6 @@ dsl_dir_tempreserve_impl(dsl_dir_t *dd, uint64_t asize, boolean_t netfree,
 	 * we're very close to full, this will allow a steady trickle of
 	 * removes to get through.
 	 */
-	uint64_t deferred = 0;
 	if (dd->dd_parent == NULL) {
 		uint64_t avail = dsl_pool_unreserved_space(dd->dd_pool,
 		    (netfree) ?
@@ -1340,7 +1339,7 @@ dsl_dir_tempreserve_impl(dsl_dir_t *dd, uint64_t asize, boolean_t netfree,
 	 */
 	if (used_on_disk + est_inflight >= quota) {
 		if (est_inflight > 0 || used_on_disk < quota ||
-		    (retval == ENOSPC && used_on_disk < quota + deferred))
+		    (retval == ENOSPC && used_on_disk < quota))
 			retval = ERESTART;
 		dprintf_dd(dd, "failing: used=%lluK inflight = %lluK "
 		    "quota=%lluK tr=%lluK err=%d\n",