fs/proc/task_mmu: Implement IOCTL to get and optionally clear info ab…

…out PTEs

This IOCTL, PAGEMAP_SCAN on pagemap file can be used to get and/or clear
the info about page table entries. The following operations are supported
in this ioctl:
- Get the information if the pages have been written-to (PAGE_IS_WRITTEN),
  file mapped (PAGE_IS_FILE), present (PAGE_IS_PRESENT) or swapped
  (PAGE_IS_SWAPPED).
- Find pages which have been written-to and write protect the pages
  (atomic PAGE_IS_WRITTEN + PAGEMAP_WP_ENGAGE)

This IOCTL can be extended to get information about more PTE bits.

Signed-off-by: Muhammad Usama Anjum <usama.anjum@collabora.com>