io_uring: check that we have a file table when allocating update slots

If IORING_FILE_INDEX_ALLOC is set asking for an allocated slot, the
helper doesn't check if we actually have a file table or not. The non
alloc path does do that correctly, and returns -ENXIO if we haven't set
one up.

Do the same for the allocated path, avoiding a NULL pointer dereference
when trying to find a free bit.

Fixes: a7c41b46 ("io_uring: let IORING_OP_FILES_UPDATE support choosing fixed file slots")
Signed-off-by: Jens Axboe <axboe@kernel.dk>