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.comAcked-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>

MAINTAINERS

@@ -6250,7 +6250,7 @@ F:	drivers/video/fbdev/s1d13xxxfb.c
 F:	include/video/s1d13xxxfb.h
 
 EROFS FILE SYSTEM
-M:	Gao Xiang <gaoxiang25@huawei.com>
+M:	Gao Xiang <xiang@kernel.org>
 M:	Chao Yu <yuchao0@huawei.com>
 L:	linux-erofs@lists.ozlabs.org
 S:	Maintained