writeback: don't warn on an unregistered BDI in __mark_inode_dirty

BDIs get unregistered during device removal, and this WARN can be
trivially triggered by hot-removing a NVMe device while running fsx
It is otherwise harmless as we still hold a BDI reference, and the
writeback has been shut down already.

Link: https://lore.kernel.org/r/20200928122613.434820-1-hch@lst.de
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jan Kara <jack@suse.cz>
diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
index e6005c7..acfb558 100644
--- a/fs/fs-writeback.c
+++ b/fs/fs-writeback.c
@@ -2321,10 +2321,6 @@
 
 			wb = locked_inode_to_wb_and_lock_list(inode);
 
-			WARN((wb->bdi->capabilities & BDI_CAP_WRITEBACK) &&
-			     !test_bit(WB_registered, &wb->state),
-			     "bdi-%s not registered\n", bdi_dev_name(wb->bdi));
-
 			inode->dirtied_when = jiffies;
 			if (dirtytime)
 				inode->dirtied_time_when = jiffies;