staging: comedi: add rw_semaphore to protect against device detachment

The 'read' and 'write' file operations on comedi devices do not use the
main mutex in the `struct comedi_device` to avoid contention with ioctls
that may take a while to complete.  However, it is necessary to protect
against the device being detached while the operation is in progress.
Add member `struct rw_semaphore attach_lock` to `struct comedi_device`
for this purpose and initialize it on creation.

The actual locking and unlocking will be implemented by subsequent
patches.  Tasks that are attaching or detaching comedi devices will
write-acquire the new semaphore whilst also holding the main mutex in
the `struct comedi_device`.  Tasks that wish to protect against the
comedi device being detached need to acquire either the main mutex, or
read-acquire the new semaphore, or both in that order.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>