pgsql: Avoid "you don't own a lock of type ExclusiveLock" in GRANT TABL

Avoid "you don't own a lock of type ExclusiveLock" in GRANT TABLESPACE.

This WARNING appeared because SearchSysCacheLocked1() read
cc_relisshared before catcache initialization, when the field is false
unconditionally. On the basis of reading false there, it constructed a
locktag as though pg_tablespace weren't relisshared. Only shared
catalogs could be affected, and only GRANT TABLESPACE was affected in
practice. SearchSysCacheLocked1() callers use one other shared-relation
syscache, DATABASEOID. DATABASEOID is initialized by the end of
CheckMyDatabase(), making the problem unreachable for pg_database.

Back-patch to v13 (all supported versions). This has no known impact
before v16, where ExecGrant_common() first appeared. Earlier branches
avoid trouble by having a separate ExecGrant_Tablespace() that doesn't
use LOCKTAG_TUPLE. However, leaving this unfixed in v15 could ensnare a
future back-patch of a SearchSysCacheLocked1() call.

Reported by Aya Iwata.

Discussion: https://postgr.es/m/OS7PR01MB11964507B5548245A7EE54E70EA212@OS7PR01MB11964.jpnprd01.prod.outlook.com

Branch
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REL_15_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/941e0c0df2bcdeb63bd822a1c46818fa9d7d8599

Modified Files
--------------
src/backend/utils/cache/syscache.c | 15 ++++++++++-----
src/test/regress/expected/tablespace.out | 5 +++++
src/test/regress/sql/tablespace.sql | 6 ++++++
3 files changed, 21 insertions(+), 5 deletions(-)