vm_object: reset iter in page_clean

Reset the page iterator unconditionally after busy_acquire(WAITFAIL)
fails.

Reported by: pho
Reported-by: syzbot+b5f9ebf4c2c63a5db681@syzkaller.appspotmail.com
Fixes: 9cc73397534e ("vm_page: use iterators in page allocaction")
Reviewed by: markj
Differential Revision: https://reviews.freebsd.org/D49904