afs: move afs_xattr_handlers to .rodata

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

Cc: David Howells <dhowells@redhat.com>
Cc: Marc Dionne <marc.dionne@auristor.com>
Cc: linux-afs@lists.infradead.org
Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>
Link: https://lore.kernel.org/r/20230930050033.41174-5-wedsonaf@gmail.com


Signed-off-by: Christian Brauner <brauner@kernel.org>