Restore the reservation of boot pages for bucket zones after r355707.

uma_startup2() sets booted = BOOT_BUCKETS after calling bucket_init(),
but before that assignment, startup_alloc() will use pages from the
reserved pool, so the bucket zones themselves are still allocated using
startup pages.

Reviewed by: rlibby
Reported by: Jenkins via lwhsu
Differential Revision: https://reviews.freebsd.org/D22797