ixgbe: fix panic when using macvlan with l2-fwd-offload enabled

Fix NULL pointer dereference in the case where a macvlan interface is
brought up while the PF is still down:

BUG: unable to handle kernel NULL pointer dereference at 0000000000000010
IP: [<ffffffffa0170fb2>] ixgbe_alloc_rx_buffers+0x42/0x1a0 [ixgbe]

Call Trace:
[<ffffffffa017336b>] ixgbe_configure_rx_ring+0x2eb/0x3d0 [ixgbe]
[<ffffffffa0173811>] ixgbe_fwd_ring_up+0xd1/0x380 [ixgbe]
[<ffffffffa0179709>] ixgbe_fwd_add+0x149/0x230 [ixgbe]
[<ffffffffa0113480>] macvlan_open+0x260/0x2b0 [macvlan]

Reported-by: Matthew Garrett <mjg59@coreos.com>
Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>