io_uring/register: add MASK support for task filter set

If IORING_REG_RESTRICTIONS_MASK is set in the flags for an
IORING_REGISTER_RESTRICTIONS_TASK operation, then further restrictions
can be added to the current set. No restrictions may be relaxed this
way. If a current set exists, the passed in set is added to the current
set. If no current set exists, the new set applied will as the current
task io_uring restriction filter.

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