dmu_tx: break tx assign/wait when pool suspends

This adjusts dmu_tx_assign/dmu_tx_wait to be interruptable if the pool
suspends while they're waiting, rather than just on the initial check
before falling back into a wait.

Since that's not always wanted, add a DMU_TX_SUSPEND flag to ignore
suspend entirely, effectively returning to the previous behaviour.

With that, it shouldn't be possible for anything with a standard
dmu_tx_assign/wait/abort loop to block under failmode=continue.

Also should be a bit tighter than the old behaviour, where a
VERIFY0(dmu_tx_assign(DMU_TX_WAIT)) could technically fail if the pool
is already suspended and failmode=continue.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Paul Dagnelie <paul.dagnelie@klarasystems.com>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Closes #17355

include/sys/dmu_tx.h

@@ -25,6 +25,7 @@
  */
 /*
  * Copyright (c) 2012, 2016 by Delphix. All rights reserved.
+ * Copyright (c) 2025, Klara, Inc.
  */
 
 #ifndef	_SYS_DMU_TX_H
@@ -80,6 +81,9 @@ struct dmu_tx {
 	/* has this transaction already been delayed? */
 	boolean_t tx_dirty_delayed;
 
+	/* whether dmu_tx_wait() should return on suspend */
+	boolean_t tx_break_on_suspend;
+
 	int tx_err;
 };