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.

Change-Id: Ibbcb570e6634e34016cd3c8d07f817f01be59210
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Git-commit: 118b23022512eb2f41ce42db70dc0568d00be4ba
Git-repo: https://git.kernel.org/cgit/linux/kernel/git/stable/linux-stable.git


Signed-off-by: Subbaraman Narayanamurthy <subbaram@codeaurora.org>