jffs2: fix sparse warning: unexpected unlock

fs/jffs2/summary.c:846:5: warning: context imbalance in 'jffs2_sum_write_sumnode' - unexpected unlock
Suggested-by: NBrian Norris <computersforpeace@gmail.com>
Suggested-by: NJosh Triplett <josh@joshtriplett.org>
Signed-off-by: NFabian Frederick <fabf@skynet.be>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>
Signed-off-by: NBrian Norris <computersforpeace@gmail.com>

fs/jffs2/summary.c

@@ -844,6 +844,7 @@ static int jffs2_sum_write_data(struct jffs2_sb_info *c, struct jffs2_eraseblock
 /* Write out summary information - called from jffs2_do_reserve_space */
 
 int jffs2_sum_write_sumnode(struct jffs2_sb_info *c)
+	__must_hold(&c->erase_completion_block)
 {
 	int datasize, infosize, padsize;
 	struct jffs2_eraseblock *jeb;