There was only one legitimate use of idr_remove_all() and a lot more of
incorrect uses (or lack of it). Now that idr_destroy() implies
idr_remove_all() and all the in-kernel users updated not to use it,
there's no reason to keep it around. Mark it deprecated so that we can
later unexport it.
idr_remove_all() is made an inline function calling __idr_remove_all()
to avoid triggering deprecated warning on EXPORT_SYMBOL().