nvme: use page-aligned buffer for identify command

Change the stack-allocated buffer for the identification command
to explicitly allocate page-aligned buffers.  Even though the spec
seems to allow having admin queue commands on non page-aligned
buffers, it seems to not be possible on my i.MX8MQ board with a
a Silicon Power P34A80.  Since all of the NVMe drivers I have seen
always do admin commands on a page-aligned buffer, which does work
on my system, it makes sense for us to do that as well.

Signed-off-by: Patrick Wildt <patrick@blueri.se>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>