btrfs: move btrfs_xattr_handlers to .rodata

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

Cc: Chris Mason <clm@fb.com>
Cc: Josef Bacik <josef@toxicpanda.com>
Cc: David Sterba <dsterba@suse.com>
Cc: linux-btrfs@vger.kernel.org
Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>
Link: https://lore.kernel.org/r/20230930050033.41174-6-wedsonaf@gmail.com


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