Fix some nits in pmap_page_array_startup().

• Use ptoa() instead of the archaic ctob().
• Use pagezero() to zero a PDP page.
• Remove PA_MIN_ADDRESS, orphaned by r351742.
• Remove unneeded parens and an unnecessary control flow statement.

Reported by: alc
Reviewed by: alc, kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D21495