MFC r263131,r263174,r263175:

  Tmpfs readdir() redundant logic and code readability cleanup.

  r263131:
    Cleanup redundant logic and add some comments to help explain how it works
    in lieu of potentially less clear code.

  r263174:
    Rename cnt to maxcookies and change its use as the condition for when to
    lookup cookies to be less obscure.

  r263175:
    Add missing FALLTHROUGH comment in tmpfs_dir_getdents for looking up '.'
    and '..'.