cxgbe(4): Avoid hang on kldunload on netlink enabled kernels.

netlink(4) calls back into the driver during detach and it attempts to
start an internal synchronized op recursively, causing an interruptible
hang. Fix it by failing the ioctl if the VI has been marked as DOOMED
by cxgbe_detach.

Here's the stack for the hang for reference.
#6 begin_synchronized_op
#7 cxgbe_media_status
#8 ifmedia_ioctl
#9 cxgbe_ioctl
#10 if_ioctl
#11 get_operstate_ether
#12 get_operstate
#13 dump_iface
#14 rtnl_handle_ifevent
#15 rtnl_handle_ifnet_event
#16 rt_ifmsg
#17 if_unroute
#18 if_down
#19 if_detach_internal
#20 if_detach
#21 ether_ifdetach
#22 cxgbe_vi_detach
#23 cxgbe_detach
#24 DEVICE_DETACH

MFC after: 3 days
Sponsored by: Chelsio Communications