staging: erofs: Replace kzalloc(struct ..) with kzalloc(*ptr)

Resolve checkpatch warning:
Prefer kzalloc(sizeof(*ptr)...) over kzalloc(sizeof(struct ..)

Signed-off-by: Shobhit Kukreti <shobhitkukreti@gmail.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Reviewed-by: Gao Xiang <gaoxiang25@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>