9p: fix fid refcount leak in v9fs_vfs_get_link

we check for protocol version later than required, after a fid has
been obtained. Just move the version check earlier.

Link: https://lkml.kernel.org/r/20220612085330.1451496-3-asmadeus@codewreck.org
Fixes: 6636b6d ("9p: add refcount to p9_fid struct")
Cc: stable@vger.kernel.org
Reviewed-by: Tyler Hicks <tyhicks@linux.microsoft.com>
Reviewed-by: Christian Schoenebeck <linux_oss@crudebyte.com>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>

Author: martinetd

fs/9p/vfs_inode.c

@@ -1251,15 +1251,15 @@ static const char *v9fs_vfs_get_link(struct dentry *dentry,
 		return ERR_PTR(-ECHILD);
 
 	v9ses = v9fs_dentry2v9ses(dentry);
-	fid = v9fs_fid_lookup(dentry);
+	if (!v9fs_proto_dotu(v9ses))
+		return ERR_PTR(-EBADF);
+
 	p9_debug(P9_DEBUG_VFS, "%pd\n", dentry);
+	fid = v9fs_fid_lookup(dentry);
 
 	if (IS_ERR(fid))
 		return ERR_CAST(fid);
 
-	if (!v9fs_proto_dotu(v9ses))
-		return ERR_PTR(-EBADF);
-
 	st = p9_client_stat(fid);
 	p9_client_clunk(fid);
 	if (IS_ERR(st))