MFC r296909:
Fix witness panic in the ipoib_ioctl() function when unloading the
ipoib module.

The bpfdetach() function is trying to turn off promiscious mode on the
network interface it is attached to while holding a mutex. The fix
consists of ignoring any further calls to the ipoib_ioctl() function
when the network interface is going to be detached. The ipoib_ioctl()
function might sleep.

Sponsored by: Mellanox Technologies