ixgbevf: Mailbox improvements

Improve reliability of the mailbox communication and remove
its potential flaws that may lead to the undefined or faulty behavior.

Recently some users reported issues on ESX with 10G Intel NICs which were
found to be caused by incorrect implementation of the PF-VF mailbox
communication.

Technical investigation highlighted areas to improve in the communication
between PF or VF that wants to send the message (sender) and the other
part which receives the message (receiver):

 - Locking the mailbox when the sender wants to send a message
 - Releasing the mailbox when the communication ends
 - Returning the result of the mailbox message execution

Signed-off-by: Radoslaw Tyl <radoslawx.tyl@intel.com>
Tested-by: Tony Brelinski <tony.brelinski@intel.com>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>