btrfs: fix transaction atomicity bug when enabling simple quotas

In the Linux kernel, the following vulnerability has been resolved:

btrfs: fix transaction atomicity bug when enabling simple quotas

Set squota incompat bit before committing the transaction that enables
the feature.

With the config CONFIG_BTRFS_ASSERT enabled, an assertion
failure occurs regarding the simple quota feature.

[5.596534] assertion failed: btrfs_fs_incompat(fs_info, SIMPLE_QUOTA), in fs/btrfs/qgroup.c:365
[5.597098] ------------[ cut here ]------------
[5.597371] kernel BUG at fs/btrfs/qgroup.c:365!
[5.597946] CPU: 1 UID: 0 PID: 268 Comm: mount Not tainted 6.13.0-rc2-00031-gf92f4749861b #146
[5.598450] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.16.2-debian-1.16.2-1 04/01/2014
[5.599008] RIP: 0010:btrfs_read_qgroup_config+0x74d/0x7a0
[5.604303] <TASK>
[5.605230] ? btrfs_read_qgroup_config+0x74d/0x7a0
[5.605538] ? exc_invalid_op+0x56/0x70
[5.605775] ? btrfs_read_qgroup_config+0x74d/0x7a0
[5.606066] ? asm_exc_invalid_op+0x1f/0x30
[5.606441] ? btrfs_read_qgroup_config+0x74d/0x7a0
[5.606741] ? btrfs_read_qgroup_config+0x74d/0x7a0
[5.607038] ? try_to_wake_up+0x317/0x760
[5.607286] open_ctree+0xd9c/0x1710
[5.607509] btrfs_get_tree+0x58a/0x7e0
[5.608002] vfs_get_tree+0x2e/0x100
[5.608224] fc_mount+0x16/0x60
[5.608420] btrfs_get_tree+0x2f8/0x7e0
[5.608897] vfs_get_tree+0x2e/0x100
[5.609121] path_mount+0x4c8/0xbc0
[5.609538] __x64_sys_mount+0x10d/0x150

The issue can be easily reproduced using the following reproducer:

root@q:linux# cat repro.sh
set -e

mkfs.btrfs -q -f /dev/sdb
mount /dev/sdb /mnt/btrfs
btrfs quota enable -s /mnt/btrfs
umount /mnt/btrfs
mount /dev/sdb /mnt/btrfs

The issue is that when enabling quotas, at btrfs_quota_enable(), we set
BTRFS_QGROUP_STATUS_FLAG_SIMPLE_MODE at fs_info->qgroup_flags and persist
it in the quota root in the item with the key BTRFS_QGROUP_STATUS_KEY, but
we only set the incompat bit BTRFS_FEATURE_INCOMPAT_SIMPLE_QUOTA after we
commit the transaction used to enable simple quotas.

This means that if after that transaction commit we unmount the filesystem
without starting and committing any other transaction, or we have a power
failure, the next time we mount the filesystem we will find the flag
BTRFS_QGROUP_STATUS_FLAG_SIMPLE_MODE set in the item with the key
BTRFS_QGROUP_STATUS_KEY but we will not find the incompat bit
BTRFS_FEATURE_INCOMPAT_SIMPLE_QUOTA set in the superblock, triggering an
assertion failure at:

btrfs_read_qgroup_config() -> qgroup_read_enable_gen()

To fix this issue, set the BTRFS_FEATURE_INCOMPAT_SIMPLE_QUOTA flag
immediately after setting the BTRFS_QGROUP_STATUS_FLAG_SIMPLE_MODE.
This ensures that both flags are flushed to disk within the same
transaction.