spa_misc: add an API for spa_namespace_lock

This is useful as debugging support, as it lets namespace lock
operations be traced directly. It will also be useful for future work to
reduce the use of spa_namespace_lock, traditionally a source of
difficult deadlocks.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Closes #17906

module/os/linux/zfs/zvol_os.c

@@ -809,8 +809,8 @@ retry:
 		 * the kernel so the only option is to return the error for
 		 * the caller to handle it.
 		 */
-		if (!mutex_owned(&spa_namespace_lock)) {
-			if (!mutex_tryenter(&spa_namespace_lock)) {
+		if (!spa_namespace_held()) {
+			if (!spa_namespace_tryenter(FTAG)) {
 				mutex_exit(&zv->zv_state_lock);
 				rw_exit(&zv->zv_suspend_lock);
 				drop_suspend = B_FALSE;
@@ -834,7 +834,7 @@ retry:
 		error = -zvol_first_open(zv, !(blk_mode_is_open_write(flag)));
 
 		if (drop_namespace)
-			mutex_exit(&spa_namespace_lock);
+			spa_namespace_exit(FTAG);
 	}
 
 	if (error == 0) {