A file supporting the memory mapping operation can be bound to a
mapper element, so that mmap() on such element's device is
transparently handed to the original file implementation.

This is particularly useful for exposing a shared memory file created
by memfd_create() into the globally visible EVL device namespace, so
that cooperating processes can find it easily without resorting to fd
inheritance or socket transmission.

Signed-off-by: Philippe Gerum <rpm@xenomai.org>