Fix another minor page deletion buffer lock issue.

Avoid accessing the leaf page's top parent tuple without a buffer lock
held during the second phase of nbtree page deletion.  The old approach
was safe, though only because VACUUM never drops its buffer pin (and
because only VACUUM itself can modify a half-dead page).  Even still, it
seems like a good idea to be strict here.  Tighten things up by copying
the top parent page's block number to a local variable before releasing
the buffer lock on the leaf page -- not after.

This is a follow-up to commit fa7ff642, which fixed a similar issue in
the first phase of nbtree page deletion.

Update some related comments in passing.

Discussion: https://postgr.es/m/CAH2-WzkLgyN3zBvRZ1pkNJThC=xi_0gpWRUb_45eexLH1+k2_Q@mail.gmail.com

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