staging: comedi: comedi_buf: factor out common code to free the async buffer

The function comedi_buf_alloc() uses two loops to free the async buffer's
buf_page_list. The first one is used at the beginning to deallocate the
current buffer. The second is used to cleanup if the new buffer allocation
fails.

Factor out the common code to a new function.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>