Properly reset the fields in clean_unrhdr.

In r324542 I neglected to reset the first and last fields of struct
unrhdr. This causes a tmpfs to fail the unr(9) consistency checks with
DIAGNOSTIC on. Fix this by resetting the fields by calling init_unrhdr.
While here, change a loop to use TAILQ_FOREACH_SAFE since it is more
readable and equally fast.

Reported by: David Wolfskill <david@catwhisker.org>
Approved by: rstone (mentor)
Sponsored by: Dell EMC Isilon