[IB] Fail sysfs queries after device is unregistered

We keep IB device structures around until the last sysfs reference is
gone, but we shouldn't ask the low-level driver to do anything after
the LLD unregisters the device.  To handle this, check the reg_state
field and just fail sysfs show() requests if the device has already
been unregistered.

Signed-off-by: Roland Dreier <rolandd@cisco.com>