NFS: Fix a bug in nfs_fscache_release_page()

commit 2c174009 upstream.

Not having an fscache cookie is perfectly valid if the user didn't mount
with the fscache option.

This patch fixes http://bugzilla.kernel.org/show_bug.cgi?id=15234



Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>