pgsql: Limit depth of forced recursion for CLOBBER_CACHE_RECURSIVELY.

Limit depth of forced recursion for CLOBBER_CACHE_RECURSIVELY.

It's somewhat surprising that we got away with this before.  (Actually,
since nobody tests this routinely AFAIK, it might've been broken for
awhile.  But it's definitely broken in the wake of commit f868a8143.)
It seems sufficient to limit the forced recursion to a small number
of levels.

Back-patch to all supported branches, like the preceding patch.

Discussion: https://postgr.es/m/12259.1532117714@sss.pgh.pa.us

Branch
------
REL_10_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/adfc156d356a6bd80293464b6a33b93c0184a92f

Modified Files
--------------
src/backend/utils/cache/inval.c | 12 +++++++++++-
1 file changed, 11 insertions(+), 1 deletion(-)