nvme-pci: fix freeing single sgl

There may only be a single DMA mapped entry from multiple physical
segments, which means we don't allocate a separte SGL list. Check the
number of allocations prior to know if we need to free something.

Freeing a single list allocation is the same for both PRP and SGL
usages, so we don't need to check the use_sgl flag anymore.

Fixes: 01df742d ("nvme-pci: remove SGL segment descriptors")
Reported-by: Niklas Schnelle <schnelle@linux.ibm.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Niklas Schnelle <schnelle@linux.ibm.com>