netdevsim: account dropped packet length in stats on queue free

Add a call to dev_dstats_rx_dropped_add() in nsim_queue_free() to
account for the number of packets dropped when purging the skb queue.
This improves the accuracy of RX drop statistics reported by netdevsim.

local_bh_{disable, enable}() protection is used to disable preemption,
which is necessary given that dev_dstats_rx_dropped_add() access
this_cpu_ptr(). See discussion in [1].

Link: https://lore.kernel.org/all/20250617055934.3fd9d322@kernel.org/

 [1]
Suggested-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://patch.msgid.link/20250618-netdevsim_stat-v4-4-19fe0d35e28e@debian.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>