libfs: Support RENAME_EXCHANGE in simple_rename()

Allow atomic exchange via RENAME_EXCHANGE when using simple_rename.
This affects binderfs, ramfs, hubetlbfs and bpffs.
Signed-off-by: Lorenz Bauer <lmb@cloudflare.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Miklos Szeredi <mszeredi@redhat.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://lore.kernel.org/bpf/20211028094724.59043-3-lmb@cloudflare.com

fs/libfs.c

@@ -479,9 +479,12 @@ int simple_rename(struct user_namespace *mnt_userns, struct inode *old_dir,
 	struct inode *inode = d_inode(old_dentry);
 	int they_are_dirs = d_is_dir(old_dentry);
 
-	if (flags & ~RENAME_NOREPLACE)
+	if (flags & ~(RENAME_NOREPLACE | RENAME_EXCHANGE))
 		return -EINVAL;
 
+	if (flags & RENAME_EXCHANGE)
+		return simple_rename_exchange(old_dir, old_dentry, new_dir, new_dentry);
+
 	if (!simple_empty(new_dentry))
 		return -ENOTEMPTY;