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 <faaland1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #7460 
Closes #7463

module/zfs/zfs_vnops.c

@@ -2273,7 +2273,7 @@ out:
  */
 /* ARGSUSED */
 int
-zfs_readdir(struct inode *ip, struct dir_context *ctx, cred_t *cr)
+zfs_readdir(struct inode *ip, zpl_dir_context_t *ctx, cred_t *cr)
 {
 	znode_t		*zp = ITOZ(ip);
 	zfsvfs_t	*zfsvfs = ITOZSB(ip);
@@ -2378,7 +2378,7 @@ zfs_readdir(struct inode *ip, struct dir_context *ctx, cred_t *cr)
 			type = ZFS_DIRENT_TYPE(zap.za_first_integer);
 		}
 
-		done = !dir_emit(ctx, zap.za_name, strlen(zap.za_name),
+		done = !zpl_dir_emit(ctx, zap.za_name, strlen(zap.za_name),
 		    objnum, type);
 		if (done)
 			break;