Further minor improvement in generic_xlog.c: always say REGBUF_STANDARD.

Since we're requiring pages handled by generic_xlog.c to be standard
format, specify REGBUF_STANDARD when doing a full-page image, so that
xloginsert.c can compress out the "hole" between pd_lower and pd_upper.
Given the current API in which this path will be taken only for a newly
initialized page, the hole is likely to be particularly large in such
cases, so that this oversight could easily be performance-significant.
I don't notice any particular change in the runtime of contrib/bloom's
regression test, though.

src/backend/access/transam/generic_xlog.c

@@ -349,7 +349,8 @@ GenericXLogFinish(GenericXLogState *state)
 			{
 				/* A full page image does not require anything special */
 				memcpy(page, pageData->image, BLCKSZ);
-				XLogRegisterBuffer(i, pageData->buffer, REGBUF_FORCE_IMAGE);
+				XLogRegisterBuffer(i, pageData->buffer,
+								   REGBUF_FORCE_IMAGE | REGBUF_STANDARD);
 			}
 			else
 			{