mac802154: add llsec structures and mutators

mac802154: add llsec structures and mutators

This patch adds containers and mutators for the major ieee802154_llsec
structures to mac802154. Most of the (rather simple) ieee802154_llsec
structs are wrapped only to provide an rcu_head for orderly disposal,
but some structs - llsec keys notably - require more complex
bookkeeping.

Since each llsec key may be referenced by a number of llsec key table
entries (with differing key ids, but the same actual key), we want to
save memory and not allocate crypto transforms for each entry in the
table. Thus, the mac802154 llsec key is reference-counted instead.
Further, each key will have four associated crypto transforms - three
CCM transforms for the authsizes 4/8/16 and one CTR transform for
unauthenticated encryption. If we had a CCM* transform that allowed
authsize 0, and authsize as part of requests instead of transforms, this
would not be necessary.

Signed-off-by: Phoebe Buckheister <phoebe.buckheister@itwm.fraunhofer.de>
Signed-off-by: David S. Miller <davem@davemloft.net>