Fix zpl_revalidate() NULL deref

In zpl_revalidate() it's possible for the nameidata to be NULL
for kernels which still accept the parameter.  In particular,
lookup_one_len() calls d_revalidate() with a NULL nameidata.

Resolve the issue by checking for a NULL nameidata in which case
just set the flags to 0.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1226

module/zfs/zpl_inode.c

@@ -396,7 +396,7 @@ static int
 #ifdef HAVE_D_REVALIDATE_NAMEIDATA
 zpl_revalidate(struct dentry *dentry, struct nameidata *nd)
 {
-	unsigned int flags = nd->flags;
+	unsigned int flags = (nd ? nd->flags : 0);
 #else
 zpl_revalidate(struct dentry *dentry, unsigned int flags)
 {