selftests: openvswitch: add dec_ttl action support and test

Add dec_ttl action support to the OVS kernel datapath selftest
framework:

  - Add dec_ttl nested NLA class to ovs-dpctl.py with proper
    OVS_DEC_TTL_ATTR_ACTION sub-attribute handling
  - Add parse support for dec_ttl(le_1(<inner_actions>)) action
    string, consistent with the odp-util.c format where le_1()
    holds the actions taken when TTL reaches 1
  - Add dpstr output formatting for dec_ttl actions
  - Add test_dec_ttl() to openvswitch.sh that verifies:
    * Normal TTL packets are forwarded after decrement
    * TTL=1 packets are dropped (TTL expiry)
    * Graceful skip via ksft_skip if kernel lacks dec_ttl support

The dec_ttl class uses late-binding type resolution to reference
ovsactions for its inner action list, avoiding circular references
at class definition time.

Signed-off-by: Minxi Hou <houminxi@gmail.com>
Reviewed-by: Aaron Conole <aconole@redhat.com>
Link: https://patch.msgid.link/20260530021443.1734484-1-houminxi@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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