vm_reserv: clarify reserv_from_object

vm_reserv_from_page returns an existing reservation, or initializes a
successor pointer, or both. If it returns an existing reservation,
callers will ignore the successor pointer, initialized or not, so
initializing it serves no purpose. So, don't initialize it in that
case, and make less ambiguous the comment that describes the
function's behavior.

Reviewed by: markj, kib, alc
Differential Revision: https://reviews.freebsd.org/D49415