In a dual processor system (2*6 cores) during IPSec throughput tests,
we see a lot of contention on the arc4 lock, used to generate the IV
of the ESP output packets.

The idea of this patch is to split this mutex in order to reduce the
contention on this lock.

Reviewed by: delphij, markm, ache
Approved by: so
Obtained from: emeric.poupon@stormshield.eu
MFC after: 1 month
Sponsored by: Stormshield
Differential Revision: https://reviews.freebsd.org/D8130