idr: Fix idr_get_next_ul race with idr_remove

Commit 5c089fd0 ("idr: Fix idr_get_next race with idr_remove")
neglected to fix idr_get_next_ul().  As far as I can tell, nobody's
actually using this interface under the RCU read lock, but fix it now
before anybody decides to use it.

Fixes: 5c089fd0 ("idr: Fix idr_get_next race with idr_remove")
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>