core: Do not fail perpetual mount units without fragment (#6459)

core: Do not fail perpetual mount units without fragment (#6459)

mount_load does not require fragment files to be present in order to
load mount units which are perpetual, or come from /proc/self/mountinfo.

mount_verify should do the same, otherwise a synthesized '-.mount' would
be marked as failed with "No such file or directory", as it is perpetual
but not marked to come from /proc/self/mountinfo at this point.

This happens for the user instance, and I suspect it was the cause of #5375
for the system instance, without gpt-generator.