vm_grab: reset iter after alloc failure

After vm_page_grab_lookup fails to find a page, and
vm_page_alloc_after fails to allocate one, reset the pages iterator
which is invalidated by a release/acquire of the object write lock
within the allocation code.

Reported by: pho
Reviewed by: kib
Fixes: 7becd87c988c ("vm_grab: use iterator for grab lookup")
Differential Revision: https://reviews.freebsd.org/D49755