commit:  cb64edb6 upstream

Above commit may introduce a race between cp_interrupt and dev_close
/ change MTU / dev_open up state. Changes cp_interrupt to tolerate
this.  Change spin_locking in cp_interrupt to avoid possible
but unobserved race.
Reported-by: N"Francois Romieu" <romieu@fr.zoreil.com>

Tested on virtual hardware, Tx MTU size up to 4096, max tx payload
    was ping -s 4068 for MTU of 4096. No real hardware, need test
    assist.
Signed-off-by: N"John Greene" <jogreene@redhat.com>
CC: "David S. Miller" <davem@davemloft.net>
CC: "David Woodhouse" <David.Woodhouse@intel.com>
Tested-by: NDavid Woodhouse <David.Woodhouse@intel.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>