bdev: Do not return EBUSY if bdev discard races with write

mainline inclusion
from mainline-5.12-rc1
commit 	767630c6
category: bugfix
bugzilla: 107770
CVE: NA

-------------------------------------------------

blkdev_fallocate() tries to detect whether a discard raced with an
overlapping write by calling invalidate_inode_pages2_range(). However
this check can give both false negatives (when writing using direct IO
or when writeback already writes out the written pagecache range) and
false positives (when write is not actually overlapping but ends in the
same page when blocksize < pagesize). This actually causes issues for
qemu which is getting confused by EBUSY errors.

Fix the problem by removing this conflicting write detection since it is
inherently racy and thus of little use anyway.

Reported-by: Maxim Levitsky <mlevitsk@redhat.com>
CC: "Darrick J. Wong" <darrick.wong@oracle.com>
Link: https://lore.kernel.org/qemu-devel/20201111153913.41840-1-mlevitsk@redhat.com


Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Baokun Li <libaokun1@huawei.com>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>