nvme-pci: code cleanup for nvme_alloc_host_mem()

Although use of for loop is preferred it is not a common practice to
have 80 char long for loop initialization and comparison section.

Use temp variables for calculating values and replace them in the
for loop with size of all variables to set to u64 since preferred
variable is declared as u64.

Signed-off-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>