client: fix ancient typo in caps revocation path

If we have dropped all references to a revoked capability, send the ack
to the MDS.  This typo has been there since v0.7 (early 2009)!

Backport: cuttlefish
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Greg Farnum <greg@inktank.com>