makefs: fix too-long swap labels

makefs: fix too-long swap labels

Apparently mkswap has it's own limit, and it seems to be one lower than the one
for ext2/3/4.

$ for i in ext2 ext3 ext4 btrfs xfs vfat swap minix; do
  echo $i && wipefs -q -a '/var/tmp/głąbźśńćąśððð.img'
  build/systemd-makefs $i '/var/tmp/głąbźśńćąśððð.img'
done

ext2
/var/tmp/głąbźśńćąśððð.img successfully formatted as ext2 (label "głąbźśńćą", uuid 7626bc5c-8ac4-43cf-87b7-1b2761272dd3)
ext3
/var/tmp/głąbźśńćąśððð.img successfully formatted as ext3 (label "głąbźśńćą", uuid 0da22cad-0dbf-4a7a-962d-12cd39d006b5)
ext4
/var/tmp/głąbźśńćąśððð.img successfully formatted as ext4 (label "głąbźśńćą", uuid dded267b-8955-4d19-82a5-1f231d446059)
btrfs
/var/tmp/głąbźśńćąśððð.img successfully formatted as btrfs (label "głąbźśńćąśððð.img", uuid 9e2e89f1-010d-4ab6-80f3-f9e215dbc225)
xfs
/var/tmp/głąbźśńćąśððð.img successfully formatted as xfs (label "głąbźśń", uuid 2cc937af-4c41-465c-8f52-aab2304bd860)
vfat
mkfs.fat 4.2 (2021-01-31)
/var/tmp/głąbźśńćąśððð.img successfully formatted as vfat (label "G__B_______", uuid a3a9e028)

swap
...
LABEL=głąbźśńć, UUID=0ab787aa-37a6-4b32-978b-d71efc6e6098
/var/tmp/głąbźśńćąśððð.img successfully formatted as swap (label "głąbźśńć", uuid 0ab787aa-37a6-4b32-978b-d71efc6e6098)

minix
...
/var/tmp/głąbźśńćąśððð.img successfully formatted as minix (no label or uuid specified)