Merge branch 'l2tp-set-l2specific_len-based-on-l2specific_type'

Lorenzo Bianconi says:

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l2tp: set l2specific_len based on l2specific_type

Do not rely on l2specific_len value provided by userspace but set sublayer
length according to l2specific_type.
Mark L2TP_ATTR_L2SPEC_LEN attribute as not used

Changes since v2:
- drop the patch related to a fix in the switch default case in
  l2tp_nl_cmd_session_create()
- use L2SPECTYPE_NONE as default case in l2tp_get_l2specific_len()

Changes since v1:
- remove l2specific_len parameter
- add sanity check on l2specific_type provided by userspace
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Signed-off-by: David S. Miller <davem@davemloft.net>