net: thunderx: avoid direct MTU assignment after WRITE_ONCE()

net: thunderx: avoid direct MTU assignment after WRITE_ONCE()

The current logic in nicvf_change_mtu() writes the new MTU to
netdev->mtu using WRITE_ONCE() before verifying if the hardware
update succeeds. However on hardware update failure, it attempts
to revert to the original MTU using a direct assignment
(netdev->mtu = orig_mtu)
which violates the intended of WRITE_ONCE protection introduced in
commit 1eb2cded45b3 ("net: annotate writes on dev->mtu from
ndo_change_mtu()")

Additionally, WRITE_ONCE(netdev->mtu, new_mtu) is unnecessarily
performed even when the device is not running.

Fix this by:
  Only writing netdev->mtu after successfully updating the hardware.
  Skipping hardware update when the device is down, and setting MTU
  directly. Remove unused variable orig_mtu.

This ensures that all writes to netdev->mtu are consistent with
WRITE_ONCE expectations and avoids unintended state corruption
on failure paths.

Signed-off-by: Alok Tiwari <alok.a.tiwari@oracle.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://patch.msgid.link/20250706194327.1369390-1-alok.a.tiwari@oracle.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>