cxl: Check if afu is not null in cxl_slbia

The pointer to an AFU in the adapter's list of AFUs can be null
if we're in the process of removing AFUs. The afu_list_lock
doesn't guard against this.

Say we have 2 slices, and we're in the process of removing cxl.
- We remove the AFUs in order (see cxl_remove). In cxl_remove_afu
for AFU 0, we take the lock, set adapter->afu[0] = NULL, and
release the lock.
- Then we get an slbia. In cxl_slbia we take the lock, and set
afu = adapter->afu[0], which is NULL.
- Therefore our attempt to check afu->enabled will blow up.

Therefore, check if afu is a null pointer before dereferencing it.

Cc: [e-mail]
Signed-off-by: Daniel Axtens <[e-mail]>
Acked-by: Michael Neuling <[e-mail]>
Acked-by: Ian Munsie <[e-mail]>
Signed-off-by: Michael Ellerman <[e-mail]>

drivers/misc/cxl/main.c

···
 		spin_lock(&adapter->afu_list_lock);
 		for (slice = 0; slice < adapter->slices; slice++) {
 			afu = adapter->afu[slice];
-			if (!afu->enabled)
+			if (!afu || !afu->enabled)
 				continue;
 			rcu_read_lock();
 			idr_for_each_entry(&afu->contexts_idr, ctx, id)