CVE-2026-64292: iommufd: Move vevent memory allocation outside spinlock

In the Linux kernel, the following vulnerability has been resolved:

iommufd: Move vevent memory allocation outside spinlock

The veventq memory allocation happens inside the spinlock. Given its depth is decided by the user space, this leaves a vulnerability, where userspace can allocate large queues to exhaust atomic memory reserves.

Move the allocation outside the spinlock and use GFPNOWAIT, which can fail fast under memory pressure without dipping into the GFPATOMIC reserves or direct-reclaiming from the threaded IRQ handler. On allocation failure, queue the losteventsheader (so userspace learns of the drop) and return -ENOMEM so the caller learns of the kernel-side memory pressure.

This is intentionally distinct from the queue-overflow path, which also queues the losteventsheader but returns 0: a full queue is an expected userspace-pacing condition rather than a kernel error.

A subsequent change will cap the upper bound of the veventqdepth.