net: dst_cache: Use nested-BH locking for dst_cache::cache

net: dst_cache: Use nested-BH locking for dst_cache::cache

dst_cache::cache is a per-CPU variable and relies on disabled BH for its
locking. Without per-CPU locking in local_bh_disable() on PREEMPT_RT
this data structure requires explicit locking.

Add a local_lock_t to the data structure and use
local_lock_nested_bh() for locking. This change adds only lockdep
coverage and does not alter the functional behaviour for !PREEMPT_RT.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://patch.msgid.link/20250512092736.229935-3-bigeasy@linutronix.de
Signed-off-by: Paolo Abeni <pabeni@redhat.com>