nvme-pci: handle changing device dma map requirements

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

nvme-pci: handle changing device dma map requirements

The initial state of dma_needs_unmap may be false, but change to true

while mapping the data iterator. Enabling swiotlb is one such case that

can change the result. The nvme driver needs to save the mapped dma

vectors to be unmapped later, so allocate as needed during iteration

rather than assume it was always allocated at the beginning. This fixes

a NULL dereference from accessing an uninitialized dma_vecs when the

device dma unmapping requirements change mid-iteration.