net/tls: narrow down the critical area of device_offload_lock

On setsockopt path we need to hold device_offload_lock from
the moment we check netdev is up until the context is fully
ready to be added to the tls_device_list.

No need to hold it around the get_netdev_for_sock().
Change the code and remove the confusing comment.

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