Close fd on error path in do_mount()

This fix closes a file descriptor before returning
error from do_mount()

Change-Id: Ib370229f16bf9ac385162d6b163cc5b079a1e73e

init/builtins.c

@@ -433,6 +433,7 @@ int do_mount(int nargs, char **args)
             sprintf(tmp, "/dev/block/loop%d", n);
             loop = open(tmp, mode);
             if (loop < 0) {
+                close(fd);
                 return -1;
             }