nfp: bpf: add simple map op cache

Each get_next and lookup call requires a round trip to the device.
However, the device is capable of giving us a few entries back,
instead of just one.

In this patch we ask for a small yet reasonable number of entries
(4) on every get_next call, and on subsequent get_next/lookup calls
check this little cache for a hit. The cache is only kept for 250us,
and is invalidated on every operation which may modify the map
(e.g. delete or update call). Note that operations may be performed
simultaneously, so we have to keep track of operations in flight.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>