io_uring: kill hot path fixed file bitmap debug checks

We test file_table.bitmap in io_file_get_fixed() to check invariants,
don't do it, it's expensive and was showing up in profiles. No reports of
this triggering has come in. Move the check to the file clear instead,
which will still catch any wrong usage.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/cf77f2ded68d2e5b2bc7355784d969837d48e023.1665891182.git.asml.silence@gmail.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>