NFS: Fabricate fscache server index key correctly

When fabricating a server index key for fscache, we should clear the index key
buffer before starting to fill it in, not in the middle.

Reported-by: James Pearson <james-p@moving-picture.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Steve Dickson <steved@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>