lockdep false positive - move txg_kick() outside of ->dp_lock

This fixes a lockdep warning by breaking a link between ->tx_sync_lock
and ->dp_lock.

The deadlock envisioned by lockdep is this:
    thread 1 holds db->db_mtx and tries to get dp->dp_lock:
	dsl_pool_dirty_space+0x70/0x2d0 [zfs]
	dbuf_dirty+0x778/0x31d0 [zfs]

    thread 2 holds bpo->bpo_lock and tries to get db->db_mtx:
        dmu_buf_will_dirty_impl
        dmu_buf_will_dirty+0x6b/0x6c0 [zfs]
        bpobj_iterate_impl+0xbe6/0x1410 [zfs]

    thread 3 holds tx->tx_sync_lock and tries to get bpo->bpo_lock:
        bpobj_space+0x63/0x470 [zfs]
        dsl_scan_active+0x340/0x3d0 [zfs]
        txg_sync_thread+0x3f2/0x1370 [zfs]

    thread 4 holds dp->dp_lock and tries to get tx->tx_sync_lock
       txg_kick+0x61/0x420 [zfs]
       dsl_pool_need_dirty_delay+0x1c7/0x3f0 [zfs]

This patch is orginally from Brian Behlendorf and slightly simplified
by me.

It breaks this cycle in thread 4 by moving the call from
dsl_pool_need_dirty_delay to txg_kick outside the section controlled
by dp->dp_lock.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Matt Ahrens <mahrens@delphix.com>
Signed-off-by: Jeff Dike <jdike@akamai.com>
Closes openzfs#9094

module/zfs/dsl_pool.c

@@ -889,14 +889,14 @@ dsl_pool_need_dirty_delay(dsl_pool_t *dp)
 	    zfs_dirty_data_max * zfs_delay_min_dirty_percent / 100;
 	uint64_t dirty_min_bytes =
 	    zfs_dirty_data_max * zfs_dirty_data_sync_percent / 100;
-	boolean_t rv;
+	uint64_t dirty;
 
 	mutex_enter(&dp->dp_lock);
-	if (dp->dp_dirty_total > dirty_min_bytes)
-		txg_kick(dp);
-	rv = (dp->dp_dirty_total > delay_min_bytes);
+	dirty = dp->dp_dirty_total;
 	mutex_exit(&dp->dp_lock);
-	return (rv);
+	if (dirty > dirty_min_bytes)
+		txg_kick(dp);
+	return (dirty > delay_min_bytes);
 }
 
 void