dmaengine: idxd: Fix ->poll() return value

dmaengine: idxd: Fix ->poll() return value

The fix to block access from different address space did not return a
correct value for ->poll() change.  kernel test bot reported that a
return value of type __poll_t is expected rather than int. Fix to return
POLLNVAL to indicate invalid request.

Fixes: 8dfa57aabff6 ("dmaengine: idxd: Fix allowing write() from different address spaces")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202505081851.rwD7jVxg-lkp@intel.com/
Signed-off-by: Dave Jiang <dave.jiang@intel.com>
Link: https://lore.kernel.org/r/20250508170548.2747425-1-dave.jiang@intel.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>