ia64: perfmon: add d_dname method

Switch ia64/perfmon to using the d_dname() instead of relying on
__d_path() to prepend the name of the root dentry to the path.

CC: Tony Luck <tony.luck@intel.com>
CC: Fenghua Yu <fenghua.yu@intel.com>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>