nfs: set verifier on existing dentries in nfs_prime_dcache

nfs_prime_dcache currently only sets the verifier when it doesn't
initially a matching dentry in the dcache. Set the verifier in the case
where we do find a dentry in the dcache. This ensures that we don't
have to look up the dentry again if we want to use it after a readdir.

Cc: Scott Mayhew <smayhew@redhat.com>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>