If netpoll attempts to use a device without polling support, it will oops
when shutting down.  This adds a check that we've actually attached to a
device.
Signed-off-by: Matt Mackall <mpm@selenic.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>