PCI/P2PDMA: Fix undefined behavior bug in struct pci_p2pdma_pagemap

PCI/P2PDMA: Fix undefined behavior bug in struct pci_p2pdma_pagemap

Struct dev_pagemap is a flexible structure, which means that it contains a
flexible-array member.  If dev_pagemap.nr_range > 1, the memory following
the dev_pagemap could be overwritten.

This is currently not an issue because pci_p2pdma_pagemap is not exposed
outside p2pdma.c, and p2pdma.c only sets dev_pagemap.nr_range to 1.

To prevent problems if p2pdma.c ever uses nr_range > 1, move the flexible
struct dev_pagemap to the end of struct pci_p2pdma_pagemap.

-Wflex-array-member-not-at-end is coming in GCC-14, and we are getting
ready to enable it globally.

Link: https://lore.kernel.org/r/ZRsUL/hATNruwtla@work
Signed-off-by: "Gustavo A. R. Silva" <gustavoars@kernel.org>
[bhelgaas: commit log]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>