Idle page tracking can also be used for process address space, not only
file mappings.

Without this change, using with '-i' option for process address space
encounters below errors reported.

  $ sudo ./page-types -p $(pidof bash) -i
  mark page idle: Bad file descriptor
  mark page idle: Bad file descriptor
  mark page idle: Bad file descriptor
  mark page idle: Bad file descriptor
  ...

Link: https://lkml.kernel.org/r/20210917032826.10669-1-changbin.du@gmail.comSigned-off-by: Changbin Du <changbin.du@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>