pgsql: pgstat: Acquire lock when reading variable-numbered stats

pgstat: Acquire lock when reading variable-numbered stats

Somewhere during the development of the patch acquiring a lock during read
access to variable-numbered stats got lost. The missing lock acquisition won't
cause corruption, but can lead to reading torn values when accessing
stats. Add the missing lock acquisitions.

Reported-by: Greg Stark <stark@mit.edu>
Reviewed-by: "Drouvot, Bertrand" <bdrouvot@amazon.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Author: Kyotaro Horiguchi <horikyota.ntt@gmail.com>
Discussion: https://postgr.es/m/CAM-w4HMYkM_DkYhWtUGV+qE_rrBxKOzOF0+5faozxO3vXrc9wA@mail.gmail.com
Backpatch: 15-

Branch
------
REL_15_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/045ec347857b66e8a494b2c12126a6d476702065

Modified Files
--------------
src/backend/utils/activity/pgstat.c       |  9 +++++++++
src/backend/utils/activity/pgstat_shmem.c | 16 ++++++++++++++++
src/include/utils/pgstat_internal.h       |  1 +
3 files changed, 26 insertions(+)