nvme-pci: compile warnings in nvme_alloc_host_mem()

"i" should be signed or it could cause a forever loop on the cleanup
path. "size" can be used uninitialized.

Fixes: 87ad72a5 ("nvme-pci: implement host memory buffer support")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>