cuse: implement memory mapping

cuse: implement memory mapping

This implements memory mapping of char devices.

Unlike memory maps for regular files this needs to allow more than one
mapping to be associated with an open device.

Tha mapping is identified by a 64bit map ID.  This is used in place of
the node ID in the STORE and RETRIEVE notifications.

Original patch by Tejun Heo.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>