uma: pcpu_page_free needs to startup_free pages from startup_alloc

After r357392, it is apparent that we do have some early-boot PCPU
zones. Make it so we can safely free pages from them if they are
actually used during early boot.

Reviewed by: jeff, markj
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D23496