cxgbe(4): Fix tracing with netlink enabled kernels.

1. The tracing ifnet's name must match the nexus name. One way to do this is to not use a unit number.

2. Do not hold the tracer lock around ether_ifattach or ether_ifdetach. netlink calls back into the driver (see stack below) and that leads to illegal lock recursion.
   
   tracer_ioctl if_ioctl get_operstate_ether get_operstate dump_iface rtnl_handle_ifevent rtnl_handle_ifattach if_attach_internal if_attach ether_ifattach t4_cloner_create

MFC after: 3 days
Sponsored by: Chelsio Communications

(cherry picked from commit e203cb393fe0b963dd585d0576dcc6a47a28c386)