Add ZIO_CHECKSUM_IS_ZERO

The ZIO_CHECKSUM_IS_ZERO macro was added in master commit:

37f8a88 Illumos 5746 - more checksumming in zfs send

That whole patch is not suitable for the release branch
but some other backported patches on that macro.

Signed-off-by: Ned Bass <bass6@llnl.gov>

include/sys/spa.h

@@ -446,6 +446,10 @@ _NOTE(CONSTCOND) } while (0)
 	((zc1).zc_word[2] - (zc2).zc_word[2]) | \
 	((zc1).zc_word[3] - (zc2).zc_word[3])))
 
+#define	ZIO_CHECKSUM_IS_ZERO(zc) \
+	(0 == ((zc)->zc_word[0] | (zc)->zc_word[1] | \
+	(zc)->zc_word[2] | (zc)->zc_word[3]))
+
 #define	DVA_IS_VALID(dva)	(DVA_GET_ASIZE(dva) != 0)
 
 #define	ZIO_SET_CHECKSUM(zcp, w0, w1, w2, w3)	\