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.deSigned-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NJan Kara <jack@suse.cz>

fs/fs-writeback.c

@@ -2321,10 +2321,6 @@ void __mark_inode_dirty(struct inode *inode, int flags)
 
 			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;