Move the redirection to stderr out of the cmd variable assignment

Putting 2>/dev/null in cmd= escapes the redirection operation, which causes
mdconfig to think it's a filename

MFC after: 2 weeks
X-MFC with: r273627
Sponsored by: EMC / Isilon Storage Division

tools/regression/zfs/misc.sh

@@ -167,8 +167,8 @@ create_memdisk()
 		if [ -n "${devname}" ]; then
 			devparam="-u ${devname}"
 		fi
-		cmd="mdconfig -a -t swap -s ${size} ${devparam} 2>/dev/null"
-		DISKNAME=`${cmd}` || die "failed: ${cmd}"
+		cmd="mdconfig -a -t swap -s ${size} ${devparam}"
+		DISKNAME=`$cmd 2>/dev/null` || die "failed: ${cmd}"
 		if [ -n "${devname}" ]; then
 			DISKNAME="${devname}"
 		fi