The new ->mmap() handler is introduced in the device operation
descriptor. This service is restricted to secondary mode, switching a
primary mode caller accordingly when invoked from an application.

The prototype is:

int mmap_handler(struct rtdm_fd *fd, struct vm_area_struct *vma);

The semantics are identical to the corresponding handler from the
regular linux file operations. The only difference is about receiving
a RTDM file descriptor (struct rtdm_fd) instead of a regular struct
file pointer. As such, a valid vma descriptor covering the target user
address space is passed to the handler, therefore common mapping
operations may be performed on it.

Two new convenience routines for mapping a chunk of kernel memory and
a physical I/O address range are provided, i.e. rtdm_mmap_kmem() and
rtdm_mmap_iomem() respectively. Both take a vma descriptor as received
by the ->mmap() handler, and the appropriate virtual/physical start
address of the memory range to map to the target address range in
user-space.

The existing rtdm_mmap_to_user() and rtdm_iomap_to_user() calls are
kept unmodified. The ->mmap() introduction has no impact on existing
drivers currently relying on these calls.