generic/250: ignore xfs_io IO errors when writing to dmerror device

generic/250: ignore xfs_io IO errors when writing to dmerror device

When testing with data=journal ext4, direct write to dmerror device
doesn't return EIO, because ext4 turns direct write to buffered
write in data=journal mode and all data is written to journal
buffer. The write only fails later when commiting journal and error
messages can be seen in dmesg.

As the test is checking on the md5 checksum of the test file, it's
ok to ignore the IO error returned by xfs_io, as long as the
checksums match the golden image.

Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>