pageinspect: Fix memory context allocation of page in brin_revmap_data()

This caused the function to fail, as the aligned copy of the raw page
given by the function caller was not saved in the correct memory
context, which needs to be multi_call_memory_ctx in this case.

Issue introduced by 076f4d9.

Per buildfarm members sifika, mylodon and longfin.  I have reproduced
that locally with macos.

Discussion: https://postgr.es/m/YjFPOtfCW6yLXUeM@paquier.xyz
Backpatch-through: 10

contrib/pageinspect/brinfuncs.c

@@ -383,15 +383,15 @@ brin_revmap_data(PG_FUNCTION_ARGS)
 		MemoryContext mctx;
 		Page		page;
 
-		/* minimally verify the page we got */
-		page = verify_brin_page(raw_page, BRIN_PAGETYPE_REVMAP, "revmap");
-
 		/* create a function context for cross-call persistence */
 		fctx = SRF_FIRSTCALL_INIT();
 
 		/* switch to memory context appropriate for multiple function calls */
 		mctx = MemoryContextSwitchTo(fctx->multi_call_memory_ctx);
 
+		/* minimally verify the page we got */
+		page = verify_brin_page(raw_page, BRIN_PAGETYPE_REVMAP, "revmap");
+
 		state = palloc(sizeof(*state));
 		state->tids = ((RevmapContents *) PageGetContents(page))->rm_tids;
 		state->idx = 0;