pgsql: Fix GIN index search for root strings in strict jsonpath queries

Fix GIN index search for root strings in strict jsonpath queries.

A jsonb_ops index scan for 'strict $ == "a scalar"' missed a matching
root string. Root scalars are stored in single-element arrays, so
gin_extract_jsonb() indexes strings with JGINFLAG_KEY. In strict mode,
jsonb_ops__extract_nodes() instead searches for JGINFLAG_STR at the
root. Leave gin_extract_jsonb() alone (else we'd force people to
reindex) and make jsonb_ops__extract_nodes() search for JGINFLAG_KEY
instead.

Author: Rui Zhao <zhaorui126(at)gmail(dot)com>
Reviewed-by: shihao zhong <zhong950419(at)gmail(dot)com>
Reviewed-by: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/CAHWVJhHH0k3PxWmS==V1p_PONOMUaSm6LjUV487fwnOqd0jTQw@mail.gmail.com
Backpatch-through: 14

Branch
------
REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/327c5f0e6c1be3494344386c0d5848c5f4ab9252

Modified Files
--------------
src/backend/utils/adt/jsonb_gin.c | 5 +++--
src/test/regress/expected/jsonb.out | 27 +++++++++++++++++++++++++++
src/test/regress/sql/jsonb.sql | 8 ++++++++
3 files changed, 38 insertions(+), 2 deletions(-)