Add ioctl to conveniently mmap a PCI device BAR into userspace.

Add the ioctl PCIOCBARMMAP on /dev/pci to conveniently create
userspace mapping of a PCI device BAR. This is enormously superior to
read the BAR value with PCIOCREAD and then try to mmap /dev/mem, and
should allow to automatically activate the mapped BARs when needed in
future.

Current implementation creates new sg pager for each user mmap
request. If the pointer (and reference) to a managed device pager is
stored in pci_map, we would be able to revoke all mappings on the BAR
deactivation or relocation. This is related to the unimplemented BAR
activation on mmap, and is postponed for the future.

Discussed with: imp, jhb
Sponsored by: The FreeBSD Foundation, Mellanox Technologies
MFC after: 2 weeks
Differential revision: https://reviews.freebsd.org/D15583