pgsql: Fix pg_blocking_pids() for relation extension locks.

Fix pg_blocking_pids() for relation extension locks.

pg_blocking_pids() assumes that members of the same lock group
never block each other, but that hasn't been true for relation
extension locks since v13. This commit teaches the function to
report lock group members as blockers for relation extension locks.
Since parallel workers are reported with their leader's PID, the
result may now include the blocked session's own PID.

Oversight in commit 85f6b49c2c.

Author: Bertrand Drouvot <bertranddrouvot(dot)pg(at)gmail(dot)com>
Reviewed-by: Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com>
Reviewed-by: Kirill Reshke <reshkekirill(at)gmail(dot)com>
Reviewed-by: Shihao Zhong <zhong950419(at)gmail(dot)com>
Discussion: https://postgr.es/m/apTpV8h%2BCYHf%2B1PJ%40bdtpg
Backpatch-through: 14

Branch
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REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/36e021cb1bb7b2e582b18467b1adca23bcf9609e

Modified Files
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doc/src/sgml/func.sgml | 21 ++++++++++++++-------
src/backend/utils/adt/lockfuncs.c | 10 ++++++++--
2 files changed, 22 insertions(+), 9 deletions(-)