VFS: Make pathwalk use d_is_reg() rather than S_ISREG()

Make pathwalk use d_is_reg() rather than S_ISREG() to determine whether to
honour O_TRUNC.  Since this occurs after complete_walk(), the dentry type
field cannot change and the inode pointer cannot change as we hold a ref on
the dentry, so this should be safe.
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/namei.c

@@ -3077,7 +3077,7 @@ static int do_last(struct nameidata *nd, struct path *path,
 	error = -ENOTDIR;
 	if ((nd->flags & LOOKUP_DIRECTORY) && !d_can_lookup(nd->path.dentry))
 		goto out;
-	if (!S_ISREG(nd->inode->i_mode))
+	if (!d_is_reg(nd->path.dentry))
 		will_truncate = false;
 
 	if (will_truncate) {