iwlwifi: mvm: properly check debugfs dentry before using it

debugfs can now report an error code if something went wrong instead of
just NULL.  So if the return value is to be used as a "real" dentry, it
needs to be checked if it is an error before dereferencing it.

This is now happening because of ff9fb72bc0 ("debugfs: return error
values, not NULL").  If multiple iwlwifi devices are in the system, this
can cause problems when the driver attempts to create the main debugfs
directory again.  Later on in the code we fail horribly by trying to
dereference a pointer that is an error value.

Reported-by: Laura Abbott <labbott@redhat.com>
Reported-by: Gabriel Ramirez <gabriello.ramirez@gmail.com>
Cc: Johannes Berg <johannes.berg@intel.com>
Cc: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Cc: Luca Coelho <luciano.coelho@intel.com>
Cc: Intel Linux Wireless <linuxwifi@intel.com>
Cc: Kalle Valo <kvalo@codeaurora.org>
Cc: stable <stable@vger.kernel.org> # 5.0
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>

drivers/net/wireless/intel/iwlwifi/mvm/debugfs-vif.c

@@ -774,6 +774,11 @@ void iwl_mvm_vif_dbgfs_register(struct iwl_mvm *mvm, struct ieee80211_vif *vif)
 		return;
 
 	mvmvif->dbgfs_dir = debugfs_create_dir("iwlmvm", dbgfs_dir);
+	if (IS_ERR_OR_NULL(mvmvif->dbgfs_dir)) {
+		IWL_ERR(mvm, "Failed to create debugfs directory under %pd\n",
+			dbgfs_dir);
+		return;
+	}
 
 	if (!mvmvif->dbgfs_dir) {
 		IWL_ERR(mvm, "Failed to create debugfs directory under %pd\n",