Implement proper support for memory map operations in the LinuxKPI,
like open, close and fault using the character device pager.

Some notes about the implementation:

1. Linux drivers set the vm_ops and vm_private_data fields during a

mmap() call to indicate that the driver wants to use the LinuxKPI VM
operations. Else these operations are not used.

2. The vm_private_data pointer is associated with a VM area structure

and inserted into an internal LinuxKPI list. If the vm_private_data
pointer already exists, the existing VM area structure is used instead
of the allocated one which gets freed.

3. The LinuxKPI's vm_private_data pointer is used as the callback

handle for the FreeBSD VM object. The VM subsystem in FreeBSD has a
similar list to identify equal handles and will only call the
character device pager's close function once.

4. All LinuxKPI VM operations are serialized through the mmap_sem

sempaphore, which is per procedure, which prevents simultaneous access
to the shared VM area structure when receiving page faults.

Obtained from: kmacy @
MFC after: 1 week
Sponsored by: Mellanox Technologies