ctf: Avoid passing a caddr_t to roundup2()

For some reason I can't reproduce this locally, but Jenkins complains.

Reported by: Jenkins
Fixes: bdf290cd3e1a ("ctf: Add v3 support to CTF tools, ctf{convert,dump,merge}")

(cherry picked from commit 3fe1f21fb364e38f2bf7e7100dad59b067b27080)