fstab-generator: ignore invalid swap priority

A failed priority is not something worth stopping boot over. Most people
have only one swap device, in which case priority is irrelevant, and even
if there is more than one swap device, they are all usable, and ignoring the
priority field should only result in some loss of performance.

The kernel will report the priority as -1 if not set, so it's easy for
people to make this mistake.

https://bugzilla.redhat.com/show_bug.cgi?id=1204336
(cherry picked from commit e0952d9d)