Bug#926735: Set severity

[Kai Lüke]

severity: serious

Maybe this can be lowered to 'important', but I believe buster should
not be released because users might misinterpret the disappearence of
the extended partition as data loss (leading to panic specially when the
GUI does not show the contained logical partitions anymore) while
actually it is not a data loss since triggering a syscall, reattaching
the device, or rebooting makes it appear again.