pgsql: Fix pg_stat_reset_single_table_counters() for shared relations

Fix pg_stat_reset_single_table_counters() for shared relations

This commit fixes the function of $subject for shared relations.  This
feature has been added by e042678.  Unfortunately, this new behavior got
removed by 5891c7a when moving statistics to shared memory.

Reported-by: Mitsuru Hinata
Author: Masahiro Ikeda
Reviewed-by: Kyotaro Horiguchi, Masahiko Sawada
Discussion: https://postgr.es/m/7cc69f863d9b1bc677544e3accd0e4b4@oss.nttdata.com
Backpatch-through: 15

Branch
------
REL_16_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/d6af45052dc8a9d97a18e1a3b9febabd891ba270

Modified Files
--------------
src/backend/utils/adt/pgstatfuncs.c |  9 ++++++--
src/test/regress/expected/stats.out | 46 +++++++++++++++++++++++++++++++++++++
src/test/regress/sql/stats.sql      | 30 ++++++++++++++++++++++++
3 files changed, 83 insertions(+), 2 deletions(-)