fstests: btrfs/226: use nodatasum mount option to prevent false alerts

fstests: btrfs/226: use nodatasum mount option to prevent false alerts

[BUG]
With recent kernel patch "btrfs: always fallback to buffered write if the
inode requires checksum", the test case btrfs/226 will fail with the
following error:

FSTYP         -- btrfs
PLATFORM      -- Linux/x86_64 btrfs-vm 6.13.0-rc6-custom+ #209 SMP PREEMPT_DYNAMIC Fri Jan 24 17:23:03 ACDT 2025
MKFS_OPTIONS  -- /dev/mapper/test-scratch1
MOUNT_OPTIONS -- /dev/mapper/test-scratch1 /mnt/scratch

btrfs/226 1s ... - output mismatch (see /home/adam/xfstests/results//btrfs/226.out.bad)
    --- tests/btrfs/226.out	2024-04-12 14:04:03.080000035 +0930
    +++ /home/adam/xfstests/results//btrfs/226.out.bad	2025-02-06 08:23:42.564298585 +1030
    @@ -39,14 +39,11 @@
     Testing write against prealloc extent at eof
     wrote 65536/65536 bytes at offset 0
     XXX Bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
    -wrote 65536/65536 bytes at offset 65536
    -XXX Bytes, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
    +pwrite: Resource temporarily unavailable
     File after write:
    ...
    (Run 'diff -u /home/adam/xfstests/tests/btrfs/226.out /home/adam/xfstests/results//btrfs/226.out.bad'  to see the entire diff)
Ran: btrfs/226
Failures: btrfs/226
Failed 1 of 1 tests

[CAUSE]
That kernel patch makes btrfs to always fallback to buffered IO if the
target inode requires data checksum.

This is to avoid more deadly problems of mismatched data checksum.

But this also means, for inodes with data checksum, RWF_NOWAIT will
always fail, because we will wait writing back the page cache, thus
breaking the RWF_NOWAIT requirement.

[FIX]
Update the test case to utilize nodatasum mount option, so that the
direct-IO will not fallback to buffered ones unconditionally.

Reported-by: Filipe Manana <fdmanana@kernel.org>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>