Fix VM buffer pin management in heap_lock_updated_tuple_rec().

Fix VM buffer pin management in heap_lock_updated_tuple_rec().

Sloppy coding in this function could lead to leaking a VM buffer pin,
or to attempting to free the same pin twice.  Repair.  While at it,
reduce the code's tendency to free and reacquire the same page pin.

Back-patch to 9.6; before that, this routine did not concern itself
with VM pages.

Amit Kapila and Tom Lane

Discussion: https://postgr.es/m/CAA4eK1KJKwhc=isgTQHjM76CAdVswzNeAuZkh_cx-6QgGkSEgA@mail.gmail.com