Replace kernel lock with mutex in ixl(4) media status.

Replace kernel lock with mutex in ixl(4) media status.

Witness found that sc_atq_mtx mutex is held when kernel lock is
acquired.  This might cause a deadlock.  Protect sc_media_status
and sc_media_active with the link state mutex instead.  Global
fields ifm->ifm_status and ifm->ifm_active are still protected by
kernel lock.

OK tobhe@