nvme: introduce alloc helper with alignment support

nvme: introduce alloc helper with alignment support

The kernel supports since v5.2 direct mapped DMA buffers to userspace.
Up to this point a bounce buffer was involved. Because the buffers are
now directly accessed by the device, the rules of alignment also apply
for the payloads.

Introduce a helper to allocate all correctly buffers aligned.

Furthermore, ensure that the buffer is a multiple of 4k, because there
are devices on the market which will always transfer a multiple of 4k,
even if we ask for less, e.g 512 bytes. This avoid stack smashes.

Signed-off-by: Daniel Wagner <dwagner@suse.de>