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 GFP_NOWAIT, which can fail
fast under memory pressure without dipping into the GFP_ATOMIC reserves or
direct-reclaiming from the threaded IRQ handler. On allocation failure,
queue the lost_events_header (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 lost_events_header 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 veventq_depth.