net/tls: simplify driver context retrieval

Currently drivers have to ensure the alignment of their tls state
structure, which leads to unnecessary layers of getters and
encapsulated structures in each driver.

Simplify all this by marking the driver state as aligned (driver_state
members are currently aligned, so no hole is added, besides ALIGN in
TLS_OFFLOAD_CONTEXT_SIZE_RX/TX would reserve this extra space, anyway.)
With that we can add a common accessor to the core.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Dirk van der Merwe <dirk.vandermerwe@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>