xattr: make the xattr array itself const

As it is currently declared, the xattr_handler structs are const but the
array containing their pointers is not. This patch makes it so that fs
modules can place them in .rodata, which makes it harder for
accidental/malicious modifications at runtime.

Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>
Link: https://lore.kernel.org/r/20230930050033.41174-2-wedsonaf@gmail.com


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