libdevdctl: Force full match of "timestamp" field name

OpenZFS generates events with a "zio_timestamp" field, which gets mistaken for
"timestamp" by libdevdctl due to imprecise string matching.  Then later it is
assumed a "timestamp" field exists when it doesn't and an exception is thrown.

Add a space to the search string so we match exactly "timestamp" rather than
anything with that as a suffix.

Approved by:	mav (mentor)
MFC after:	3 days
Sponsored by:	iXsystems, Inc.

lib/libdevdctl/event.cc

@@ -427,7 +427,7 @@ Event::TimestampEventString(std::string &eventString)
 		 * Add a timestamp as the final field of the event if it is
 		 * not already present.
 		 */
-		if (eventString.find("timestamp=") == string::npos) {
+		if (eventString.find(" timestamp=") == string::npos) {
 			const size_t bufsize = 32;	// Long enough for a 64-bit int
 			timeval now;
 			char timebuf[bufsize];