PCI/P2PDMA: Constify 'struct bin_attribute'

The sysfs core now allows instances of 'struct bin_attribute' to be
moved into read-only memory. Make use of that to protect them against
accidental or malicious modifications.

Link: https://lore.kernel.org/r/20241202-sysfs-const-bin_attr-pci-v1-3-c32360f495a7@weissschuh.net


Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Logan Gunthorpe <logang@deltatee.com>