smb: client: fix regression with guest option

When mounting a CIFS share with 'guest' mount option, mount.cifs(8)
will set empty password= and password2= options.  Currently we only
handle empty strings from user= and password= options, so the mount
will fail with

	cifs: Bad value for 'password2'

Fix this by handling empty string from password2= option as well.

Link: https://bbs.archlinux.org/viewtopic.php?id=303927
Reported-by: Adam Williamson <awilliam@redhat.com>
Closes: https://lore.kernel.org/r/83c00b5fea81c07f6897a5dd3ef50fd3b290f56c.camel@redhat.com
Fixes: 35f8342 ("smb3: fix broken reconnect when password changing on the server by allowing password rotation")
Cc: stable@vger.kernel.org
Signed-off-by: Paulo Alcantara (Red Hat) <pc@manguebit.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

Author: Paulo Alcantara

fs/smb/client/fs_context.c

@@ -171,6 +171,7 @@ const struct fs_parameter_spec smb3_fs_parameters[] = {
 	fsparam_string("username", Opt_user),
 	fsparam_string("pass", Opt_pass),
 	fsparam_string("password", Opt_pass),
+	fsparam_string("pass2", Opt_pass2),
 	fsparam_string("password2", Opt_pass2),
 	fsparam_string("ip", Opt_ip),
 	fsparam_string("addr", Opt_ip),
@@ -1131,6 +1132,9 @@ static int smb3_fs_context_parse_param(struct fs_context *fc,
 		} else if (!strcmp("user", param->key) || !strcmp("username", param->key)) {
 			skip_parsing = true;
 			opt = Opt_user;
+		} else if (!strcmp("pass2", param->key) || !strcmp("password2", param->key)) {
+			skip_parsing = true;
+			opt = Opt_pass2;
 		}
 	}