Fix locking issue with fixed-size stats template in injection_points

Hi all,

While doing some concurrency benchmarking with injection_points stats
enabled in a server, I have been able to trigger an assertion failure
in pgstat_begin_changecount_write():
#4 0x0000564917fdc816 in ExceptionalCondition
(conditionName=0x7f60af0dc5c8 "(*cc & 1) == 0",
fileName=0x7f60af0dc598
"../../../../src/include/utils/pgstat_internal.h",
lineNumber=831) at assert.c:65
#5 0x00007f60af0da3ef in pgstat_begin_changecount_write
(cc=0x7f60acbb8e10) at
../../../../src/include/utils/pgstat_internal.h:831
#6 0x00007f60af0db1d9 in pgstat_report_inj_fixed (numattach=0,
numdetach=0, numrun=1, numcached=0, numloaded=0) at
injection_stats_fixed.c:155
#7 0x00007f60af0d8b5c in injection_points_run (fcinfo=0x564931b66588)
at injection_points.c:429

This can be reproduced as follows. First, postgresql.conf:
shared_preload_libraries = 'injection_points'
injection_points.stats = on

Then something like the following command:
$ cat create_inj.sql
\set id random(1,100000)
select injection_points_attach('popo:id', 'notice');
select injection_points_run('popo:id');
select injection_points_detach('popo:id');
$ pgbench -n -T 300 -f create_inj.sql -c 10

The failure is not surprising, because the stats reports can happen in
a concurrent fashion when a point is run for example, contrary to
other fixed-sized stats kind where the reports are only done by a
single process (archiver, bgwriter, checkpointer). So this is just a
matter of acquiring a lock that was forgotten, to make sure that the
changes are consistent. Far from critical as this is template code,
still embarrassing.

Thoughts or comments?
--
Michael