ixgbevf: Reduce size of maximum rx buffer

There's no need to support up to 15k buffers since the HW is limited to
9.5k in SR-IOV mode.  Instead, allocate buffers that fit and align inside
of a 32K memory buffer.

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>
Signed-off-by: David S. Miller <davem@davemloft.net>