Move _bt_upgrademetapage() into critical section.

Teodor Sigaev <teodor@sigaev.ru>

Commit: 08186dc05b3eed9c1f29b3349b122487d84ef3eb
Author: Teodor Sigaev <teodor@sigaev.ru>
Date: 2018-05-30T16:45:39Z
Releases: 11.0
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

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