Previously start_port and end_port were defined in 2 places, cache.c and
device.c and this prevented their use in other modules.

Make these common functions, change the name to reflect the rdma
name space, and update existing users.
Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>