VFS: unify old_mnt_idmap and new_mnt_idmap in renamedata

A rename operation can only rename within a single mount.  Callers of
vfs_rename() must and do ensure this is the case.

So there is no point in having two mnt_idmaps in renamedata as they are
always the same.  Only one of them is passed to ->rename in any case.

This patch replaces both with a single "mnt_idmap" and changes all
callers.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: NeilBrown <neil@brown.name>
Signed-off-by: Christian Brauner <brauner@kernel.org>