squashfs: move squashfs_xattr_handlers to .rodata

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

Cc: Phillip Lougher <phillip@squashfs.org.uk>
Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>
Link: https://lore.kernel.org/r/20230930050033.41174-25-wedsonaf@gmail.com


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