qmempool: add support for free in softirq context

Adding qmempool_free_softirq() function call support.  This
is not yet used in the upstream kernel, but it allow us to
micro benchmark the benefit.

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
qmempool: benchmark the benefit of calling qmempool from softirq context

This allow is to benchmark the benefit of invoking qmempool from
softirq context, without actually running in softirq.  As this
saves us the local_bh_{disable,enable} calls, and cost of
checking in_serving_softirq().

Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>