pgsql: Fix initialization of fake LSN for unlogged relations

Fix initialization of fake LSN for unlogged relations

9155580 has changed the value of the first fake LSN for unlogged
relations from 1 to FirstNormalUnloggedLSN (aka 1000), GiST requiring a
non-zero LSN on some pages to allow an interlocking logic to work, but
its value was still initialized to 1 at the beginning of recovery or
after running pg_resetwal.  This fixes the initialization for both code
paths.

Author: Takayuki Tsunakawa
Reviewed-by: Dilip Kumar, Kyotaro Horiguchi, Michael Paquier
Discussion: https://postgr.es/m/OSBPR01MB2503CE851940C17DE44AE3D9FE6F0@OSBPR01MB2503.jpnprd01.prod.outlook.com
Backpatch-through: 12

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/9f0172bba75028a7a5b1a470406adcc2af0f3b86

Modified Files
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src/backend/access/transam/xlog.c | 2 +-
src/bin/pg_resetwal/pg_resetwal.c | 2 +-
2 files changed, 2 insertions(+), 2 deletions(-)