net: stmmac: Improve Tx timer arm logic further

Calling hrtimer_start() on an already-active txtimer is unnecessary
and expensive. Skip the restart if the timer is already active by
adding an hrtimer_active() check before hrtimer_start().

Previously, each packet reset the timer to tx_coal_timer in the future,
acting as a sliding window that delayed NAPI under burst traffic. With
this change, an already-active timer is left to fire sooner, scheduling
NAPI within tx_coal_timer of the first packet and freeing TX descriptors
earlier.

There is no race concern: hrtimer_start() is internally serialized and
safe to call on an active timer. In the event of a race between
hrtimer_active() and hrtimer_start(), the worst case is calling
hrtimer_start() on an already-active timer, which is identical to the
pre-patch behaviour.

Performance on Cyclone V with dwmac-socfpga (iperf3 -u -b 0 -l 64):
  Before: ~45200 pps
  After:  ~52300 pps (~15% improvement)

Additionally, ~10% improvement in UDP throughput observed on Agilex5,
with hrtimer CPU usage reduced from ~8% to ~0.6%.

Signed-off-by: Rohan G Thomas <rohan.g.thomas@altera.com>
Tested-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Signed-off-by: Nazim Amirul <muhammad.nazim.amirul.nazle.asmade@altera.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/20260529064659.32287-1-muhammad.nazim.amirul.nazle.asmade@altera.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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