9p/fs: add MODULE_DESCIPTION

Fix modpost warning that MODULE_DESCRIPTION is missing in fs/9p/9p.o

Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>
Message-ID: <20231025223107.1274963-1-asmadeus@codewreck.org>
Reviewed-by: Christian Schoenebeck <linux_oss@crudebyte.com>