sequences: Lock buffer before initializing page

fill_seq_fork_with_data(), used to initialize a new sequence relation, only
locked the buffer after calling PageInit(), even though PageInit() modifies
page contents.

This is unlikely to cause real-world issues, as the relation is exclusively
locked at that point, and the buffer not yet marked dirty, so other processes
should not access the buffer.

This issue looks to have been present since the introduction of sequences in
e8647c45d66a.

Given the low risk, it does not seem worth backpatching the fix.

Discussion: https://postgr.es/m/20230404185501.wdkmo3k7bedlx6qk@awork3.anarazel.de

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