NVMe: Enable BUILD_BUG_ON checks

Since _nvme_check_size() wasn't being called from anywhere, the compiler
was optimising it away ... along with all the link-time build failures
that would result if any of the structures were the wrong size.  Call it
from nvme_exit() for no particular reason.

Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>