elf: Fix comments and logic in _dl_add_to_slotinfo

Since

  commit a509eb117f
  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).

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

elf/dl-tls.c

@@ -998,16 +998,7 @@ _dl_add_to_slotinfo (struct link_map *l, bool do_add)
 		+ TLS_SLOTINFO_SURPLUS * sizeof (struct dtv_slotinfo));
       if (listp == NULL)
 	{
-	  /* We ran out of memory.  We will simply fail this
-	     call but don't undo anything we did so far.  The
-	     application will crash or be terminated anyway very
-	     soon.  */
-
-	  /* We have to do this since some entries in the dtv
-	     slotinfo array might already point to this
-	     generation.  */
-	  ++GL(dl_tls_generation);
-
+	  /* We ran out of memory while resizing the dtv slotinfo list.  */
 	  _dl_signal_error (ENOMEM, "dlopen", NULL, N_("\
 cannot create TLS data structures"));
 	}