Provide the unique implementation for the VOP_GETPAGES() method used
by ffs and ext2fs. Remove duplicated call to vm_page_zero_invalid(),
done by VOP and by vm_pager_getpages(). Use vm_pager_free_nonreq().

Reviewed by: alc (previous version)
Sponsored by: The FreeBSD Foundation
MFC after: 6 weeks (after r271596)