tsnep: fix timestamping with a stacked DSA driver

This driver is susceptible to a form of the bug explained in commit
c26a2c2ddc01 ("gianfar: Fix TX timestamping with a stacked DSA driver")
and in Documentation/networking/timestamping.rst section "Other caveats
for MAC drivers", specifically it timestamps any skb which has
SKBTX_HW_TSTAMP, and does not consider if timestamping has been enabled
in adapter->hwtstamp_config.tx_type.

Evaluate the proper TX timestamping condition only once on the TX
path (in tsnep_xmit_frame_ring()) and store the result in an additional
TX entry flag. Evaluate the new TX entry flag in the TX confirmation path
(in tsnep_tx_poll()).

This way SKBTX_IN_PROGRESS is set by the driver as required, but never
evaluated. SKBTX_IN_PROGRESS shall not be evaluated as it can be set
by a stacked DSA driver and evaluating it would lead to unwanted
timestamps.

Fixes: 403f69bbdbad ("tsnep: Add TSN endpoint Ethernet MAC driver")
Suggested-by: Vladimir Oltean <[email protected]>
Signed-off-by: Gerhard Engleder <[email protected]>
Reviewed-by: Vladimir Oltean <[email protected]>
Link: https://patch.msgid.link/[email protected]
Signed-off-by: Jakub Kicinski <[email protected]>

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