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

mainline inclusion
from mainline-v5.11-rc1
commit f7387170
category: bugfix
bugzilla: 109280
CVE: NA

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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>

conflicts:
fs/fs-writeback.c

Signed-off-by: Ye Bin <yebin10@huawei.com>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>