Fix double-XLogBeginInsert call in GIN page splits.

If data checksums or wal_log_hints is on, and a GIN page is split, the code
to find a new, empty, block was called after having already called
XLogBeginInsert(). That causes an assertion failure or PANIC, if finding the
new block involves updating a FSM page that had not been modified since last
checkpoint, because that update is WAL-logged, which calls XLogBeginInsert
again. Nested XLogBeginInsert calls are not supported.

To fix, rearrange GIN code so that XLogBeginInsert is called later, after
finding the victim buffers.

Reported by Jeff Janes.

src/backend/access/gin/ginentrypage.c

@@ -557,6 +557,7 @@ entryPlaceToPage(GinBtree btree, Buffer buf, GinBtreeStack *stack,
 		data.isDelete = insertData->isDelete;
 		data.offset = off;
 
+		XLogBeginInsert();
 		XLogRegisterBuffer(0, buf, REGBUF_STANDARD);
 		XLogRegisterBufData(0, (char *) &data,
 							offsetof(ginxlogInsertEntry, tuple));