Add d_clear_d_op() compatibility

Added d_clear_d_op() helper function which clears some flags and the
registered dentry->d_op table.  This is required because d_set_d_op()
issues a warning when the dentry operations table is already set.
For the .zfs control directory to work properly we must be able to
override the default operations table and register custom .d_automount
and .d_revalidate callbacks.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ned Bass <bass6@llnl.gov>
Closes openzfs#1230

include/linux/dcache_compat.h

@@ -60,4 +60,24 @@ d_set_d_op(struct dentry *dentry, dentry_operations_t *op)
 }
 #endif /* HAVE_D_SET_D_OP */
 
+/*
+ * 2.6.38 API addition,
+ * Added d_clear_d_op() helper function which clears some flags and the
+ * registered dentry->d_op table.  This is required because d_set_d_op()
+ * issues a warning when the dentry operations table is already set.
+ * For the .zfs control directory to work properly we must be able to
+ * override the default operations table and register custom .d_automount
+ * and .d_revalidate callbacks.
+ */
+static inline void
+d_clear_d_op(struct dentry *dentry)
+{
+#ifdef HAVE_D_SET_D_OP
+	dentry->d_op = NULL;
+	dentry->d_flags &=
+	    ~(DCACHE_OP_HASH | DCACHE_OP_COMPARE |
+	      DCACHE_OP_REVALIDATE | DCACHE_OP_DELETE);
+#endif /* HAVE_D_SET_D_OP */
+}
+
 #endif /* _ZFS_DCACHE_H */

module/zfs/zpl_ctldir.c

@@ -267,6 +267,7 @@ zpl_snapdir_lookup(struct inode *dip, struct dentry *dentry,
 		return ERR_PTR(error);
 
 	ASSERT(error == 0 || ip == NULL);
+	d_clear_d_op(dentry);
 	d_set_d_op(dentry, &zpl_dops_snapdirs);
 
 	return d_splice_alias(ip, dentry);
@@ -370,6 +371,7 @@ zpl_snapdir_mkdir(struct inode *dip, struct dentry *dentry, zpl_umode_t mode)
 
 	error = -zfsctl_snapdir_mkdir(dip, dname(dentry), vap, &ip, cr, 0);
 	if (error == 0) {
+		d_clear_d_op(dentry);
 		d_set_d_op(dentry, &zpl_dops_snapdirs);
 		d_instantiate(dentry, ip);
 	}