pgsql: Move I/O before the index_update_stats() buffer lock region.

Move I/O before the index_update_stats() buffer lock region.

Commit a07e03fd8fa7daf4d1356f7cb501ffe784ea6257 enlarged the work done
here under the pg_class heap buffer lock.  Two preexisting actions are
best done before holding that lock.  Both RelationGetNumberOfBlocks()
and visibilitymap_count() do I/O, and the latter might exclusive-lock a
visibility map buffer.  Moving these reduces contention and risk of
undetected LWLock deadlock.  Back-patch to v12, like that commit.

Discussion: https://postgr.es/m/20241031200139.b4@rfd.leadboat.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/b412f402d1e020c5dac94f3bf4a005db69519b99

Modified Files
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src/backend/catalog/index.c | 62 +++++++++++++++++++++++++++++----------------
1 file changed, 40 insertions(+), 22 deletions(-)