staging: erofs: code cleanup for option parsing of fault_injection

staging: erofs: code cleanup for option parsing of fault_injection

Define a dummpy function of erofs_build_fault_attr() when macro
CONFIG_EROFS_FAULT_INJECTION is disabled, so that we don't have to
check the macro in calling place. Based on above adjustment,
do proper code cleanup for option parsing of fault_injection.

Signed-off-by: Chengguang Xu <cgxu519@gmx.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Reviewed-by: Gao Xiang <gaoxiang25@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>