io_uring: fix fget/fput handling

This isn't a straight port of commit 84c4e1f for aio.c, since
io_uring doesn't use files in exactly the same way. But it's pretty
close. See the commit message for that commit.

This essentially fixes a use-after-free with the poll command
handling, but it takes cue from Linus's approach to just simplifying
the file handling. We move the setup of the file into a higher level
location, so the individual commands don't have to deal with it. And
then we release the reference when we free the associated io_kiocb.

Fixes: 221c5eb ("io_uring: add support for IORING_OP_POLL")
Signed-off-by: Jens Axboe <axboe@kernel.dk>