barrier: Don't return -EOPNOTSUPP to the caller if the device does not support barriers

barrier: Don't return -EOPNOTSUPP to the caller if the device does not support barriers

The caller cannot really do much about the situation anyway. Instead log
a warning if this is the first such failed barrier we see, so that the
admin can look into whether this poses a data integrity problem or not.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>