cxl/region: Do not try to cleanup after cxl_region_setup_targets() fails

In the Linux kernel, the following vulnerability has been resolved:

cxl/region: Do not try to cleanup after cxl_region_setup_targets() fails

Commit 5e42bcbc3fef ("cxl/region: decrement ->nr_targets on error in
cxl_region_attach()") tried to avoid 'eiw' initialization errors when
->nr_targets exceeded 16, by just decrementing ->nr_targets when
cxl_region_setup_targets() failed.

Commit 86987c766276 ("cxl/region: Cleanup target list on attach error")
extended that cleanup to also clear cxled->pos and p->targets[pos]. The
initialization error was incidentally fixed separately by:
Commit 8d4285425714 ("cxl/region: Fix port setup uninitialized variable
warnings") which was merged a few days after 5e42bcbc3fef.

But now the original cleanup when cxl_region_setup_targets() fails
prevents endpoint and switch decoder resources from being reused:

1) the cleanup does not set the decoder's region to NULL, which results
in future dpa_size_store() calls returning -EBUSY
2) the decoder is not properly freed, which results in future commit
errors associated with the upstream switch

Now that the initialization errors were fixed separately, the proper
cleanup for this case is to just return immediately. Then the resources
associated with this target get cleanup up as normal when the failed
region is deleted.

The ->nr_targets decrement in the error case also helped prevent
a p->targets[] array overflow, so add a new check to prevent against
that overflow.

Tested by trying to create an invalid region for a 2 switch * 2 endpoint
topology, and then following up with creating a valid region.