igbvf: add lock around mailbox ops

The PF driver assumes the VF will not send another mailbox message until
the PF has written its reply to the previous message.  If the VF does,
that message will be silently dropped by the PF before it writes its
reply to the mailbox.  This results in a VF mailbox timeout for posted
messages waiting for an ACK, and the VF is reset by the
igbvf_watchdog_task in the VM.

Add a lock around the VF mailbox ops to prevent the VF from sending
another message while the PF is still processing the previous one.

Signed-off-by: Greg Edwards <gedwards@ddn.com>
Tested-by: Aaron Brown <aaron.f.brown@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>