idpf: do not linearize big TSO packets

idpf: do not linearize big TSO packets

idpf has a limit on number of scatter-gather frags
that can be used per segment.

Currently, idpf_tx_start() checks if the limit is hit
and forces a linearization of the whole packet.

This requires high order allocations that can fail
under memory pressure. A full size BIG-TCP packet
would require order-7 alocation on x86_64 :/

We can move the check earlier from idpf_features_check()
for TSO packets, to force GSO in this case, removing the
cost of a big copy.

This means that a linearization will eventually happen
with sizes smaller than one MSS.

__idpf_chk_linearize() is renamed to idpf_chk_tso_segment()
and moved to idpf_lib.c

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Przemek Kitszel <przemyslaw.kitszel@intel.com>
Cc: Jacob Keller <jacob.e.keller@intel.com>
Cc: Madhu Chittim <madhu.chittim@intel.com>
Cc: Pavan Kumar Linga <pavan.kumar.linga@intel.com>
Cc: Willem de Bruijn <willemb@google.com>
Cc: Andrew Lunn <andrew+netdev@lunn.ch>
Reviewed-by: Joshua Hay <joshua.a.hay@intel.com>
Tested-by: Brian Vazquez <brianvv@google.com>
Acked-by: Tony Nguyen <anthony.l.nguyen@intel.com>
Link: https://patch.msgid.link/20250818195934.757936-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>