Ignore *.o.ur-safe build artifacts

Generated when building on Ubuntu 18.04.  Also ignore the new
dynamically generated zfs-mount-generator.8 man page, and the
module/.cache.mk file.

Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #7534

module/.gitignore

@@ -6,6 +6,7 @@
 .*.cmd
 .*.d
 
+/.cache.mk
 /.tmp_versions
 /Module.markers
 /Module.symvers

module/Makefile.in

@@ -54,6 +54,8 @@ clean:
 	if [ -f @LINUX_SYMBOLS@ ]; then $(RM) @LINUX_SYMBOLS@; fi
 	if [ -f Module.markers ]; then $(RM) Module.markers; fi
 
+	find . -name '*.ur-safe' -type f -print | xargs $(RM)
+
 modules_install:
 	@# Install the kernel modules
 	$(MAKE) -C @LINUX_OBJ@ SUBDIRS=`pwd` $@ \