m_uiotombuf_nomap(): Stop clearing PG_ZERO in newly allocated pages

The caller should not be passing M_ZERO in the first place, so PG_ZERO
will not be preserved by the page allocator and clearing it accomplishes
nothing.

Reviewed by: gallatin, jhb
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D28808