afs: Fix misplaced inc of net->cells_outstanding

afs: Fix misplaced inc of net->cells_outstanding

Fix net->cells_outstanding being incremented before the check for failure
of idr_alloc_cyclic(), leaving the count incremented on error.

Fixes: 88c853c3f5c0 ("afs: Fix cell refcounting by splitting the usage counter")
Reported-by: Hillf Danton <hdanton@sina.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://patch.msgid.link/20260622090856.2746629-12-dhowells@redhat.com
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>