nvme-pci: use correct size to free the hmb buffer

dev->host_mem_size value is updated only after the successful buffer
allocation of hmb descriptor. Otherwise, it may have some undefined value.
So, use the correct size to free the hmb buffer when the hmb descriptor
buffer allocation failed.

Signed-off-by: Francis Pravin <francis.p@samsung.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Keith Busch <kbusch@kernel.org>