Don't try to re-insert an already present but invalid page.

This could happen if a thread doing a page-in loses a ZFS range lock
race to a thread writing to the same range

This fixes "panic: vm_page_alloc: pindex already allocated" in
http://docs.FreeBSD.org/cgi/mid.cgi?1372165971.96049.42.camel

Submitted by: avg
MFC after: 1 week