gfs2_drevalidate(): use stable parent inode and name passed by caller

No need to mess with dget_parent() for the former; for the latter we really should
not rely upon ->d_name.name remaining stable.  Theoretically a UAF, but it's
hard to exfiltrate the information...

Reviewed-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>