NFS: Don't require a filehandle to refresh the inode in nfs_prime_dcache()

If the server does not return a valid set of attributes that we can
use to either create a file or refresh the inode, then there is no
value in calling nfs_prime_dcache().

However if we're just refreshing the inode using the attributes that
the server returned, then it shouldn't matter whether or not we have
a filehandle, as long as we check the fsid+fileid combination.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>