The maximum label length is specified by UEFI and enforced/asserted by
systemd-repart. This lets evaluation fail already and give the user
some more information about what's wrong.
Also warn when the suggested label length is exceeded. This serves as a
safety mechanism for using systemd-sysupdate style A/B updates where the
version number is encoded in the label and might not be incrementable
when the maximum label size is reached.
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