Add "access" function pointer to the "vm_operations_struct" structure
in the LinuxKPI. While at it document when to use the "virtual_address" or
the "address" field in the "vm_fault" structure.

Submitted by: Johannes Lundberg <johalun0@gmail.com>
MFC after: 1 week
Sponsored by: Mellanox Technologies
Sponsored by: Limelight Networks