netfilter: nf_tables: set element timeout update support

Store new timeout and expiration in transaction object, use them to
update elements from .commit path. Otherwise, discard update if .abort
path is exercised.

Use update_flags in the transaction to note whether the timeout,
expiration, or both need to be updated.

Annotate access to timeout extension now that it can be updated while
lockless read access is possible.

Reject timeout updates on elements with no timeout extension.

Element transaction remains in the 96 bytes kmalloc slab on x86_64 after
this update.

This patch requires ("netfilter: nf_tables: use timestamp to check for
set element timeout") to make sure an element does not expire while
transaction is ongoing.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>