cifs: rename cifs_readdir_lookup to cifs_prime_dcache and make it void return

The caller doesn't do anything with the dentry, so there's no point in
holding a reference to it on return. Also cifs_prime_dcache better
describes the actual purpose of the function.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <smfrench@gmail.com>