Implement poll() method for debugfs and be sure that the
debug_data won't be freed on ir or on read().
With this change, poll() will return POLLIN if either data was
filled or if data was read. That allows read() to return 0
to indicate EOF in the latter case.
As poll() is now provided, fix support for non-block mode.
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>