RHEL 7.5 compat: FMODE_KABI_ITERATE

As of RHEL 7.5 the mainline fops.iterate() method was added to
the file_operations structure.

This wouldn't have been a problem however in order to maintain
KABI compatibility it was added to the very end of the structure.
Callers intending to use this extended interface were additionally
required to set the FMODE_KABI_ITERATE flag on the file struct
when opening the directory.

Update the ZPL to set this RHEL specific flag when required.
Using this interface will mean that kmods built for RHEL 7.5
will not work on 7.4 without being rebuilt.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue openzfs#7460

module/zfs/zpl_file.c

@@ -92,6 +92,22 @@ zpl_iterate(struct file *filp, struct dir_context *ctx)
 	return (error);
 }
 
+#if defined(HAVE_VFS_ITERATE) && defined(FMODE_KABI_ITERATE)
+/*
+ * RHEL 7.5 compatibility; the fops.iterate() method was added to
+ * the file_operations structure but in order to maintain KABI
+ * compatibility all callers must set FMODE_KABI_ITERATE which
+ * is checked in iterate_dir().
+ */
+static int
+zpl_dir_open(struct inode *ip, struct file *filp)
+{
+	filp->f_mode |= FMODE_KABI_ITERATE;
+
+	return (0);
+}
+#endif
+
 #if !defined(HAVE_VFS_ITERATE) && !defined(HAVE_VFS_ITERATE_SHARED)
 static int
 zpl_readdir(struct file *filp, void *dirent, filldir_t filldir)
@@ -885,6 +901,9 @@ const struct file_operations zpl_file_operations = {
 const struct file_operations zpl_dir_file_operations = {
 	.llseek		= generic_file_llseek,
 	.read		= generic_read_dir,
+#if defined(HAVE_VFS_ITERATE) && defined(FMODE_KABI_ITERATE)
+	.open		= zpl_dir_open,
+#endif
 #ifdef HAVE_VFS_ITERATE_SHARED
 	.iterate_shared	= zpl_iterate,
 #elif defined(HAVE_VFS_ITERATE)