add simple_dname()

From:

commit 118b2302
Author:     Al Viro <viro@zeniv.linux.org.uk>
AuthorDate: Sat Aug 24 12:08:17 2013 -0400
Commit:     Al Viro <viro@zeniv.linux.org.uk>
CommitDate: Sat Aug 24 12:10:17 2013 -0400

    cope with potentially long ->d_dname() output for shmem/hugetlb

    dynamic_dname() is both too much and too little for those - the
    output may be well in excess of 64 bytes dynamic_dname() assumes
    to be enough (thanks to ashmem feeding really long names to
    shmem_file_setup()) and vsnprintf() is an overkill for those
    guys.

    Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>