loop: remove the racy bd_inode->i_mapping->nrpages asserts

Nothing prevents a file system or userspace opener of the block device
from redirtying the page right afte sync_blockdev returned.  Fortunately
data in the page cache during a block device change is mostly harmless
anyway.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jan Kara <jack@suse.cz>
Tested-by: Darrick J. Wong <djwong@kernel.org>
Link: https://lore.kernel.org/r/20220330052917.2566582-9-hch@lst.de


Signed-off-by: Jens Axboe <axboe@kernel.dk>