readdir: make sure to verify directory entry for legacy interfaces too

This does the directory entry name verification for the legacy
"fillonedir" (and compat) interface that goes all the way back to the
dark ages before we had a proper dirent, and the readdir() system call
returned just a single entry at a time.

Nobody should use this interface unless you still have binaries from
1991, but let's do it right.

This came up during discussions about unsafe_copy_to_user() and proper
checking of all the inputs to it, as the networking layer is looking to
use it in a few new places.  So let's make sure the _old_ users do it
all right and proper, before we add new ones.

See also commit 8a23eb804ca4 ("Make filldir[64]() verify the directory
entry filename is valid") which did the proper modern interfaces that
people actually use. It had a note:

    Note that I didn't bother adding the checks to any legacy interfaces
    that nobody uses.

which this now corrects.  Note that we really don't care about POSIX and
the presense of '/' in a directory entry, but verify_dirent_name() also
ends up doing the proper name length verification which is what the
input checking discussion was about.

[ Another option would be to remove the support for this particular very
  old interface: any binaries that use it are likely a.out binaries, and
  they will no longer run anyway since we removed a.out binftm support
  in commit eac616557050 ("x86: Deprecate a.out support").

  But I'm not sure which came first: getdents() or ELF support, so let's
  pretend somebody might still have a working binary that uses the
  legacy readdir() case.. ]

Link: https://lore.kernel.org/lkml/CAHk-=wjbvzCAhAtvG0d81W5o0-KT5PPTHhfJ5ieDFq+bGtgOYg@mail.gmail.com/
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

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