9p: move xattr-related structs to .rodata

9p: move xattr-related structs to .rodata

This makes it harder for accidental or malicious changes to
v9fs_xattr_user_handler, v9fs_xattr_trusted_handler,
v9fs_xattr_security_handler, or v9fs_xattr_handlers at runtime.

Cc: Eric Van Hensbergen <ericvh@kernel.org>
Cc: Latchesar Ionkov <lucho@ionkov.net>
Cc: Dominique Martinet <asmadeus@codewreck.org>
Cc: Christian Schoenebeck <linux_oss@crudebyte.com>
Cc: v9fs@lists.linux.dev
Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>
Link: https://lore.kernel.org/r/20230930050033.41174-4-wedsonaf@gmail.com
Acked-by: Dominique Martinet <asmadeus@codewreck.org>
Reviewed-by: Christian Schoenebeck <linux_oss@crudebyte.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>