btrfs: label should not contain return char

generally if you use
  echo "test" > /sys/fs/btrfs/<fsid>/label
it would introduce return char at the end and it can not
be part of the label. The correct command is
  echo -n "test" > /sys/fs/btrfs/<fsid>/label

This patch will check for this user error

Signed-off-by: Anand Jain <Anand.Jain@oracle.com>
Signed-off-by: Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
Reviewed-by: Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
Tested-by: Satoru Takeuchi <takeuchi_satoru@jp.fujitsu.com>
Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>