io_uring: add validate_fixed_range() for validate fixed buffer

Add helper of validate_fixed_range() for validating fixed buffer
range.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Caleb Sander Mateos <csander@purestorage.com>
Link: https://lore.kernel.org/r/20250325135155.935398-2-ming.lei@redhat.com


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