elf: Fix comments and logic in _dl_add_to_slotinfo

Since

  commit a509eb11
  Avoid late dlopen failure due to scope, TLS slotinfo updates [BZ #25112]

the generation counter update is not needed in the failure path.
That commit ensures allocation in _dl_add_to_slotinfo happens before
the demarcation point in dlopen (it is called twice, first time is for
allocation only where dlopen can still be reverted on failure, then
second time actual dtv updates are done which then cannot fail).

--
v2:
- expanded the commit message a bit.