Literally follow POSIX:

If the bs= expr operand is specified and no conversions other than sync,
noerror, or notrunc are requested, the data returned from each input
block shall be written as a separate output block.

In particular, when both bs=size and conv=sparce were specified, the
resulted file was fully filled, instead of sparce.

PR:	standards/177742
Submitted by:	Matthew Rezny <mrezny@hexaneinc.com>
MFC after:	2 weeks

bin/dd/dd.c

@@ -358,7 +358,7 @@ dd_in(void)
 		 * than noerror, notrunc or sync are specified, the block
 		 * is output without buffering as it is read.
 		 */
-		if (ddflags & C_BS) {
+		if ((ddflags & ~(C_NOERROR | C_NOTRUNC | C_SYNC)) == C_BS) {
 			out.dbcnt = in.dbcnt;
 			dd_out(1);
 			in.dbcnt = 0;