net/sched: act_mirred: better wording on protection against excessive stack growth

with commit e2ca070f ("net: sched: protect against stack overflow in
TC act_mirred"), act_mirred protected itself against excessive stack growth
using per_cpu counter of nested calls to tcf_mirred_act(), and capping it
to MIRRED_RECURSION_LIMIT. However, such protection does not detect
recursion/loops in case the packet is enqueued to the backlog (for example,
when the mirred target device has RPS or skb timestamping enabled). Change
the wording from "recursion" to "nesting" to make it more clear to readers.

CC: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Reviewed-by: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Acked-by: Jamal Hadi Salim <jhs@mojatatu.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>