Fix termination state for idr_for_each_entry_ul()

The comment for idr_for_each_entry_ul() states

  after normal termination @entry is left with the value NULL

This is not correct in the case where UINT_MAX has an entry in the idr.
In that case @entry will be non-NULL after termination.
No current code depends on the documentation being correct, but to
save future code we should fix it.

Also fix idr_for_each_entry_continue_ul().  While this is not documented
as leaving @entry as NULL, the mellanox driver appears to depend on
it doing so.  So make that explicit in the documentation as well as in
the code.

Fixes: e33d2b74 ("idr: fix overflow case for idr_for_each_entry_ul()")
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Chris Mi <chrism@mellanox.com>
Cc: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: David S. Miller <davem@davemloft.net>