xfs: check for shutdown before going to sleep in xfs_select_zone

Ensure the file system hasn't been shut down before waiting for a free
zone to become available, because that won't happen on a shut down
file system.  Without this processes can occasionally get stuck in
the allocator wait loop when racing with a file system shutdown.
This sporadically happens when running generic/388 or generic/475.

Fixes: 4e4d520755 ("xfs: add the zoned space allocator")
Reported-by: Shinichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hans Holmberg <hans.holmberg@wdc.com>
Tested-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>

fs/xfs/xfs_zone_alloc.c

@@ -727,7 +727,7 @@ xfs_select_zone(
 	for (;;) {
 		prepare_to_wait(&zi->zi_zone_wait, &wait, TASK_UNINTERRUPTIBLE);
 		oz = xfs_select_zone_nowait(mp, write_hint, pack_tight);
-		if (oz)
+		if (oz || xfs_is_shutdown(mp))
 			break;
 		schedule();
 	}