ixgbevf: Add lock around mailbox ops to prevent simultaneous access

This change adds a spinlock around the mailbox accesses to prevent
simultaneous access to the mailboxes.

Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Greg Rose <gregory.v.rose@intel.com>
Tested-by: Sibai Li <sibai.li@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>