LINUX: fix RedHat 7.5 ENOTDIR issues

LINUX: fix RedHat 7.5 ENOTDIR issues

Red Hat Linux 7.5 beta introduces a new file->f_mode flag
FMODE_KABI_ITERATE as a means for certain in-tree filesystems to
indicate that they have implemented file operation iterate() instead of
readdir().  The kernel routine iterate_dir() tests this flag to decide
whether to invoke the file operation iterate() or readdir().

The OpenAFS configure script detects that the file operation iterate()
is available under RH7.5 and so implements iterate() as
afs_linux_readdir().  However, since OpenAFS does not set
FMODE_KABI_ITERATE on any of its files, the kernel's iterate_dir() will
not invoke iterate() for any OpenAFS files.  OpenAFS has also not
implemented readdir(), so iterate_dir() must return -ENOTDIR.

Instead, modify OpenAFS to fall back to readdir() in this case.

Change-Id: I242276150ab2a506e1e9c5c752e3f17d36c98935
Reviewed-on: https://gerrit.openafs.org/12935
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>