vm_map: fix iterator jump size

The index value in the loop in vm_map_pmap_enter jumps by 1, or some
superpage size, in each iteration. Jump by the superpage size only
when the entire superpage is being mapped.

Reported by: pho
Reported-by: syzbot+1cc9ede76727d2ea2e8d@syzkaller.appspotmail.com
Reviewed by: alc, kib, markj
Tested by: pho
Fixes: Fixes: b3d89a0cde94 ("vm_map: use page iterators in pmap_enter")
Differential Revision: https://reviews.freebsd.org/D49987