staging: erofs: code cleanup for erofs_kmalloc()

Define a dummy function of time_to_inject()/erofs_show_injection_info(),
so that we don't have to check macro CONFIG_EROFS_FAULT_INJECTION in
calling place.

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