MAINTAINERS: erofs: update my email address

This email address will not be available in a few days.
Update my own email address to xiang@kernel.org, which
should be available all the time.

Link: https://lore.kernel.org/r/20200328040036.117974-1-gaoxiang25@huawei.com
Acked-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>