mutex: add __fio_mutex_remove()

We have to remember to free the condvar, so add a __fio_mutex_remove()
for cases that init a mutex inside an smalloc'ed region with
__fio_mutex_init().

Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/filelock.c b/filelock.c
index b252a97..18e8875 100644
--- a/filelock.c
+++ b/filelock.c
@@ -144,10 +144,11 @@
 
 	ff = fio_hash_find(hash);
 	if (ff) {
-		ff->references--;
+		int refs = --ff->references;
 		fio_mutex_up(&ff->lock);
-		if (!ff->references) {
+		if (!refs) {
 			flist_del(&ff->list);
+			__fio_mutex_remove(&ff->lock);
 			sfree(ff);
 		}
 	} else