injection_points: Add proper locking when reporting fixed-variable stats

injection_points: Add proper locking when reporting fixed-variable stats

Contrary to its siblings for the archiver, the bgwriter and the
checkpointer stats, pgstat_report_inj_fixed() can be called
concurrently.  This was causing an assertion failure, while messing up
with the stats.

This code is aimed at being a template for extension developers, so it
is not a critical issue, but let's be correct.  This module has also
been useful for some benchmarking, at least for me, and that was how I
have discovered this issue.

Oversight in f68cd847fa40.

Author: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: wenhui qiu <qiuwenhuifx@gmail.com>
Discussion: https://postgr.es/m/aNnXbAXHPFUWPIz2@paquier.xyz
Backpatch-through: 18