MFV/ZoL: Fix wrong assertion in libzfs diff error handling

In compare(), all error cases set the error code to EPIPE, so when an
error is set, the correct assertion to make is that the error is EPIPE,
not EINVAL.

Reviewed-by: Richard Elling <Richard.Elling@RichardElling.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@freqlabs.com>
Closes #8743
zfsonlinux/zfs@9dc41a769d

Submitted by:	Ryan Moeller <ryan@freqlabs.com>
MFC after:	1 week
Sponsored by:	iXsystems, Inc.
Differential Revision:	https://reviews.freebsd.org/D20118

cddl/contrib/opensolaris/lib/libzfs/common/libzfs_diff.c

@@ -472,7 +472,7 @@ differ(void *arg)
 	if (err)
 		return ((void *)-1);
 	if (di->zerr) {
-		ASSERT(di->zerr == EINVAL);
+		ASSERT(di->zerr == EPIPE);
 		(void) snprintf(di->errbuf, sizeof (di->errbuf),
 		    dgettext(TEXT_DOMAIN,
 		    "Internal error: bad data from diff IOCTL"));