Acquire spinlock when updating 2PC slot data during logical decoding creation

The creation of a logical decoding context in CreateDecodingContext()
updates some data of its slot for two-phase transactions if enabled by
the caller, but the code forgot to acquire a spinlock when updating
these fields like any other code paths.  This could lead to the read of
inconsistent data.

Oversight in a8fd13c.

Author: Sawada Masahiko
Discussion: https://postgr.es/m/CAD21AoAD8_fp47191LKuecjDd3DYhoQ4TaucFco1_TEr_jQ-Zw@mail.gmail.com
Backpatch-through: 15

src/backend/replication/logical/logical.c

@@ -555,8 +555,10 @@ CreateDecodingContext(XLogRecPtr start_lsn,
 	/* Mark slot to allow two_phase decoding if not already marked */
 	if (ctx->twophase && !slot->data.two_phase)
 	{
+		SpinLockAcquire(&slot->mutex);
 		slot->data.two_phase = true;
 		slot->data.two_phase_at = start_lsn;
+		SpinLockRelease(&slot->mutex);
 		ReplicationSlotMarkDirty();
 		ReplicationSlotSave();
 		SnapBuildSetTwoPhaseAt(ctx->snapshot_builder, start_lsn);