net/9p/client.c: put refcount of trans_mod in error case in parse_opts()

In my testing, the second mount will fail after umounting successfully.
The reason is that we put refcount of trans_mod in the correct case
rather than the error case in parse_opts() at last.  That will cause the
refcount decrease to -1, and when we try to get trans_mod again in
try_module_get(), we could only increase refcount to 0 which will cause
failure as follows:

parse_opts
  v9fs_get_trans_by_name
    try_module_get : return NULL to caller which cause error

So we should put refcount of trans_mod in error case.

Link: http://lkml.kernel.org/r/[email protected]
Fixes: 9421c3e ("net/9p/client.c: fix potential refcnt problem of trans module")
Signed-off-by: Jun Piao <[email protected]>
Reviewed-by: Yiwen Jiang <[email protected]>
Reviewed-by: Greg Kurz <[email protected]>
Reviewed-by: Dominique Martinet <[email protected]>
Tested-by: Dominique Martinet <[email protected]>
Cc: Eric Van Hensbergen <[email protected]>
Cc: Ron Minnich <[email protected]>
Cc: Latchesar Ionkov <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: JunPiaoHW

net/9p/client.c

@@ -225,7 +225,8 @@ static int parse_opts(char *opts, struct p9_client *clnt)
 	}
 
 free_and_return:
-	v9fs_put_trans(clnt->trans_mod);
+	if (ret)
+		v9fs_put_trans(clnt->trans_mod);
 	kfree(tmp_options);
 	return ret;
 }