Optimize zfsd for the happy case

If there are no damaged pools, then ignore all GEOM events.  We only use
them to fix damaged pools.  However, still pay attention to ZFS events.

MFC after:	20 days
X-MFC-With:	329284
Sponsored by:	Spectra Logic Corp

cddl/usr.sbin/zfsd/case_file.cc

@@ -186,6 +186,12 @@ CaseFile::DeSerialize()
 	free(caseFiles);
 }
 
+bool
+CaseFile::Empty()
+{
+	return (s_activeCases.empty());
+}
+
 void
 CaseFile::LogAll()
 {

cddl/usr.sbin/zfsd/case_file.h

@@ -134,6 +134,11 @@ class CaseFile
 	 */
 	static void      DeSerialize();
 
+	/**
+	 * \brief returns true if there are no CaseFiles
+	 */
+	static bool	Empty();
+
 	/**
 	 * \brief Emit syslog data on all active CaseFile%%s in the system.
 	 */

cddl/usr.sbin/zfsd/zfsd_event.cc

@@ -98,8 +98,16 @@ bool
 GeomEvent::Process() const
 {
 	/*
-	 * We are only concerned with create arrivals and physical path changes,
-	 * because those can be used to satisfy online and autoreplace operations
+	 * We only use GEOM events to repair damaged pools.  So return early if
+	 * there are no damaged pools
+	 */
+	if (CaseFile::Empty())
+		return (false);
+
+	/*
+	 * We are only concerned with arrivals and physical path changes,
+	 * because those can be used to satisfy online and autoreplace
+	 * operations
 	 */
 	if (Value("type") != "GEOM::physpath" && Value("type") != "CREATE")
 		return (false);