Putting the cpumask on the stack is deprecated for a long time (since
2d3854a3), as these can be big. Given that, change the on-stack
allocation of allowed_mask to be dynamically allocated.

Fixes: f011c9cf ("io_uring/sqpoll: do not allow pinning outside of cpuset")
Signed-off-by: Felix Moessbauer <felix.moessbauer@siemens.com>
Link: https://lore.kernel.org/r/20240916111150.1266191-1-felix.moessbauer@siemens.comSigned-off-by: Jens Axboe <axboe@kernel.dk>