MAINTAINERS: 9p: add Christian Schoenebeck as reviewer

Volunteering as reviewer for 9p patches. As I am quite familiar with the
9p code base in the Linux kernel already, plus being current maintainer
of 9p in QEMU this move probably makes sense.

Link: https://lkml.kernel.org/r/E1n4jXv-000445-GK@lizzy.crudebyte.com
Signed-off-by: Christian Schoenebeck <linux_oss@crudebyte.com>
[Dominique: reworded description]
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>

Author: cschoenebeck

MAINTAINERS

@@ -226,6 +226,7 @@ F:	drivers/net/ethernet/8390/
 M:	Eric Van Hensbergen <ericvh@gmail.com>
 M:	Latchesar Ionkov <lucho@ionkov.net>
 M:	Dominique Martinet <asmadeus@codewreck.org>
+R:	Christian Schoenebeck <linux_oss@crudebyte.com>
 L:	v9fs-developer@lists.sourceforge.net
 S:	Maintained
 W:	http://swik.net/v9fs