Staging: hv: handle skb allocation failure

Some fixes to receive handling:
  * Dieing with assertion failure when running out of memory is not ok
  * Use newer alloc function to get aligned skb
  * Dropped statistic is supposed to be incremented only by
    driver it was responsible for the drop.

Compile tested only.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Cc: Hank Janssen <hjanssen@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>