net/core: protect users of napi_alloc_cache against reentrance

On -RT the code running in BH can not be moved to another CPU so CPU
local variable remain local. However the code can be preempted
and another task may enter BH accessing the same CPU using the same
napi_alloc_cache variable.
This patch ensures that each user of napi_alloc_cache uses a local lock.

Cc: stable-rt@vger.kernel.org
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>