Remove block number field from nbtree stack.

Remove block number field from nbtree stack.

The initial value of the nbtree stack downlink block number field
recorded during an initial descent of the tree wasn't actually used.
Both _bt_getstackbuf() callers overwrote the value with their own value.

Remove the block number field from the stack struct, and add a child
block number argument to _bt_getstackbuf() in its place.  This makes the
overall design of _bt_getstackbuf() clearer.

Author: Peter Geoghegan
Reviewed-By: Anastasia Lubennikova
Discussion: http://postgr.es/m/CAH2-Wzmx+UbXt2YNOUCZ-a04VdXU=S=OHuAuD7Z8uQq-PXTYUg@mail.gmail.com