pgsql: Move _bt_upgrademetapage() into critical section.

Move _bt_upgrademetapage() into critical section.

Any changes on page should be done in critical section, so move
_bt_upgrademetapage into critical section. Improve comment. Found by Amit
Kapila during post-commit review of 857f9c36.

Author: Amit Kapila

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/08186dc05b3eed9c1f29b3349b122487d84ef3eb

Modified Files
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src/backend/access/nbtree/nbtinsert.c | 8 ++++----
src/backend/access/nbtree/nbtpage.c   | 8 ++++----
src/include/access/nbtree.h           | 2 +-
3 files changed, 9 insertions(+), 9 deletions(-)