fs/9p: Make sure we properly instantiate dentry.

For lookup if we get ENOENT error from the server we still
instantiate the dentry. We need to make sure we have dentry
operations set in that case so that a later dput on the dentry
does the expected. Without the patch we get the below error

#ln  -sf abc abclink
ln: creating symbolic link `abclink': No such file or directory

Now on the host do
$ touch abclink

Guest now gives ENOENT error.
# ls
ls: cannot access abclink: No such file or directory

Debugged-by: Gautham R Shenoy <ego@in.ibm.com>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Eric Van Hensbergen <ericvh@gmail.com>