memory: add file backed pages

[d4ilyrun]

This issue proposes adding support for file backed pages to the memory subsystem.

A file mapped page refers to a memory page that is mapped directly to a file, allowing the contents of the file to be accessed via memory rather than explicit I/O operations.

TODO

• Change vm_normal segment driver to vm_vnode
• Associate vm_segment with a vnode + offset
• Fill page with the vnode's content during lazy loading phase
• Make physical page CoW by default

References

• SunOS VI - Virtual Memory Implementation