"make commit-prereq" should clean up rocksjava properly

Summary: "make commit-prereq" fails to clean up java, which can cause rocksjava failure.

Test Plan: Run commit-prepreq

Reviewers: IslamAbdelRahman, rven, kradhakrishnan, yhchiang

Reviewed By: yhchiang

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D43575

Makefile

@@ -1062,7 +1062,7 @@ jdb_bench:
 
 commit-prereq:
 	$(MAKE) clean && $(MAKE) all check;
-	$(MAKE) clean && $(MAKE) rocksdbjava;
+	$(MAKE) clean && $(MAKE) jclean && $(MAKE) rocksdbjava;
 	$(MAKE) clean && USE_CLANG=1 $(MAKE) all;
 	$(MAKE) clean && OPT=-DROCKSDB_LITE $(MAKE) static_lib;