btrfs: allow btrfs_truncate_block() to fallback to nocow for data space reservation

mainline inclusion
from mainline-v5.13-rc5
commit 6d4572a9
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I39MZM
CVE: NA

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

[BUG]
When the data space is exhausted, even if the inode has NOCOW attribute,
we will still refuse to truncate unaligned range due to ENOSPC.

The following script can reproduce it pretty easily:
	#!/bin/bash

	dev=/dev/test/test
	mnt=/mnt/btrfs

	umount $dev &> /dev/null
	umount $mnt &> /dev/null

	mkfs.btrfs -f $dev -b 1G
	mount -o nospace_cache $dev $mnt
	touch $mnt/foobar
	chattr +C $mnt/foobar

	xfs_io -f -c "pwrite -b 4k 0 4k" $mnt/foobar > /dev/null
	xfs_io -f -c "pwrite -b 4k 0 1G" $mnt/padding &> /dev/null
	sync

	xfs_io -c "fpunch 0 2k" $mnt/foobar
	umount $mnt

Currently this will fail at the fpunch part.

[CAUSE]
Because btrfs_truncate_block() always reserves space without checking
the NOCOW attribute.

Since the writeback path follows NOCOW bit, we only need to bother the
space reservation code in btrfs_truncate_block().

[FIX]
Make btrfs_truncate_block() follow btrfs_buffered_write() to try to
reserve data space first, and fall back to NOCOW check only when we
don't have enough space.

Such always-try-reserve is an optimization introduced in
btrfs_buffered_write(), to avoid expensive btrfs_check_can_nocow() call.

This patch will export check_can_nocow() as btrfs_check_can_nocow(), and
use it in btrfs_truncate_block() to fix the problem.

Reference: https://patchwork.kernel.org/project/linux-btrfs/patch/20200130052822.11765-1-wqu@suse.com


Reported-by: Martin Doucha <martin.doucha@suse.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Conflicts:
	fs/btrfs/file.c
	fs/btrfs/inode.c
Signed-off-by: Gou Hao <gouhao@uniontech.com>
Signed-off-by: Cheng Jian <cj.chengjian@huawei.com>
Reviewed-by: Jiao Fenfang <jiaofenfang@uniontech.com>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>