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
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D28808

(cherry picked from commit 608c44f96e88f28f9607374a6c9327d13d3d3d0e)