RHEL 7.5 compat: FMODE_KABI_ITERATE

As of RHEL 7.5 the mainline fops.iterate() method was added to
the file_operations structure and is correctly detected by the
configure script.

Normally this is what we want, but in order to maintain KABI
compatibility the RHEL change additionally does the following:

* Requires that callers intending to use this extended interface
  set the FMODE_KABI_ITERATE flag on the file structure when
  opening the directory.
* Adds the fops.iterate() method to the end of the structure,
  without removing fops.readdir().

This change updates the configure check to ignore the RHEL 7.5+
variant of fops.iterate() when detected.  Instead fallback to
the fops.readdir() interface which will be available.

Finally, add the 'zpl_' prefix to the directory context wrappers
to avoid colliding with the kernel provided symbols when both
the fops.iterate() and fops.readdir() are provided by the kernel.

Reviewed-by: Olaf Faaland <[email protected]>
Reviewed-by: Tony Hutter <[email protected]>
Signed-off-by: Brian Behlendorf <[email protected]>
Closes #7460 
Closes #7463 

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