btrfs: sysfs: convert remaining scnprintf to sysfs_emit

The sysfs_emit is the safe API for writing to the sysfs files,
previously converted from scnprintf, there's one left to do in
btrfs_read_policy_show.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>