ceph: move ceph_xattr_handlers to .rodata

ceph: move ceph_xattr_handlers to .rodata

This makes it harder for accidental or malicious changes to
ceph_xattr_handlers at runtime.

Cc: Xiubo Li <xiubli@redhat.com>
Cc: Ilya Dryomov <idryomov@gmail.com>
Cc: Jeff Layton <jlayton@kernel.org>
Cc: ceph-devel@vger.kernel.org
Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>
Link: https://lore.kernel.org/r/20230930050033.41174-7-wedsonaf@gmail.com
Acked-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>