Avoid -Wconversion warnings when using checksum_impl.h

This does not matter much when compiling Postgres proper as many
warnings exist when enabling this compilation flag, but it can be
annoying for external modules willing to use both.

Author: David Steele
Discussion: https://postgr.es/m/91d86c8a-11fc-7b88-43eb-5ca3f6fb8bd3@pgmasters.net

src/include/storage/checksum_impl.h

@@ -211,5 +211,5 @@ pg_checksum_page(char *page, BlockNumber blkno)
 	 * Reduce to a uint16 (to fit in the pd_checksum field) with an offset of
 	 * one. That avoids checksums of zero, which seems like a good idea.
 	 */
-	return (checksum % 65535) + 1;
+	return (uint16) ((checksum % 65536) + 1);
 }