Allow user space to create XRC domains.  Because XRCDs are expected to
be shared among multiple processes, we use inodes to identify an XRCD.

Based on patches by Jack Morgenstein <jackm@dev.mellanox.co.il>
Signed-off-by: NSean Hefty <sean.hefty@intel.com>
Signed-off-by: NRoland Dreier <roland@purestorage.com>