nvme: fix another 32-bit build warning

nvme: fix another 32-bit build warning

The nvme_user_cmd function was recently moved around from one file
to another, which made a warning reappear that I had fixed before
at some point:

drivers/nvme/host/core.c: In function 'nvme_user_cmd':
drivers/nvme/host/core.c:424:4: warning: cast to pointer from integer of different size [-Wint-to-pointer-cast]

This applies the same workaround that we have elsewhere in the
driver with an extra type cast to uintptr_t.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 1673f1f08c88 ("nvme: move block_device_operations and ns/ctrl freeing to common code")
Link: https://lkml.org/lkml/2015/10/9/611
Signed-off-by: Jens Axboe <axboe@fb.com>