Properly reset fields in clean_unrhdr.

[mjoras]

Summary

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 to be as if init_unrhdr
was called.

Test Plan

Tested using a tmpfs with an extracted ports tree, unmounted with DIAGNOSTIC
on.

[mjoras]

• use init_unrhdr instead of duplicating it

[mjoras]

• Might as well use TAILQ_FOREACH_SAFE while we are here.