nfs: move nfs4_xattr_handlers to .rodata

This makes it harder for accidental or malicious changes to
nfs4_xattr_handlers at runtime.

Cc: Trond Myklebust <trond.myklebust@hammerspace.com>
Cc: Anna Schumaker <anna@kernel.org>
Cc: linux-nfs@vger.kernel.org
Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>
Link: https://lore.kernel.org/r/20230930050033.41174-19-wedsonaf@gmail.com


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